In Wells hola! is rendered 'hey! stop!' It is defined in PIV 2005 as
Interj. por veki la atenton aŭ alvoki: ~!Colin,Colin!
I translate: 'an interjection for rousing attention or calling: Hey! Colin, Colin!'.

In a place I worked a Mexican woman would often greet people with 'Hola!' which might be the way that 'Hey!' is used in place of 'Hello!' in North America. Whether this kind of usage is likely to be adopted in Esperantujo will be likely to depend on whether it has a wide enough international appeal.

I have seen “Hola” in an old Esperanto book of short plays, so it must be quite early.

So it's acceptable usage, if a little obscure then?

I am cool with that. I just didn't want to start using it and find out it was a neologism or worse, a krokodila word

Couple of hits in Tekstaro too..

Telstaro:1 trovo en La skandalo pro Jozefo
..gesinjoraj moŝtoj — ĉu neniu el inter vi deziras sonĝprofeti al si?! Hola, hola, mondumaj grandsinjoroj, noblaj sinjorinoj, ĉarmaj virgulinoj — degnu a..

1 trovo en Mortula ŝipo
..dioj estas mi sen sci’. Inferajn punojn ridas mi. Hola hej, sur alta mar’, hola, mortular’!

Ironchef:I don't have a PIV, so I can't verify..... anyone? I like the idea of using "Hola!" in Esperanto; it's a far less formal greeting than using Saluton! all the time.

According to PIV2002, "hola" is not a greeting per se but more of an interjection to get someone's attention:

PIV2002:hola! Interj., por veki la atenton aŭ alvoki: ~! Colin, Colin! Z

tommjames:Both hola and ha lo are mentioned in PMEG.

La duvorta alvoko ha lo estas ofte misskribata *halo*, kio devus elparoliĝi *hAlo* (same kiel la O-vorto halo). Ha lo estas tamen ĉiam elparolata “ha lO”. Ankaŭ ne eblas skribi *halo’* kun apostrofo, ĉar ĝi neniel estas O-vorto.

Hola estas ofte elparolata “holA”, kaj devas tiam esti skribata ho la°.

That part of PMEG always seemed odd to me; I have heard those interjections with the accent on the first syllable plenty of times, so I would disagree that "Ha lo estas tamen ĉiam elparolata “ha lO”."

Similarly he seems to think that "OK" (okej) is always pronounced with the second syllable accented, when I personally see a lot of variation and intonation depending on speaker and specific situation and nuance, with both O-kay and o-KAY occurring frequently.

And furthermore, it’s (in my opinion) silly to say that an interjection must follow the Esperanto rules of accentuation. Besides, why can’t one just write haló?

Because then you are using a symbol that is unknown in Esperanto, and does not even have a consistent meaning in other languages.

From the Esperanto Wikipedia (translated by yours truly for the English forum):

There are different ways to show accent in written Esperanto. Because in Esperanto words the accent is fixed, a general convention has not yet been developed, although it is needed for those times when it is necessary to show the accent in pronunciation guides.

The following systems are used:

[LISTO]Not marking an accent on the next-to-last syllable, and marking it on the last with an apostrophe at the end of the word: Oĝalan’. With this system it is not clear how to mark [the accent in] other situations. Some Esperanto learning guides recommend this system.[/list]

(This one makes no sense to me…)

[LISTO]Marking the accented syllable with capital letters: OĝaLAN. The Esperanto Wikipedia (including this article) uses this system.[/list]

(OK, that’s a bit ugly, but with a professional small-cap font it might work.)

[LISTO]A similar system, with only the accented vowel marked with a capital letter: OĝalAn or more often oĝalAn. This system, however, can be misleading when the first syllable is accented, or when two vowels (such as a diphthong) are accented.[/list]

(This doesn’t work very well if the word is a proper noun, as is very likely—although, like the previous system, it might work in professionally typeset material.)

[LISTO]To mark the accent with an acute diacritic above the accented vowel: Oĝalán. Several Esperanto periodicals and Esperanto learning guides (such as the famous Journey in Esperanto-land by Boris Kolker) use this system. The system looks good enough to use it in printed text, but sometimes people accuse it of not being neutral—the accute diacritic marks the accent in Spanish, Russian and several other languages, but in languages such as Czech, it indicates a change in the quality of the vowel, not the accent.[/list]

(Add to the first set of languages Dutch, and to the second French. Having English as my mother-tongue—a language that does not use the acute except in loanwords—I can vouch that I always viewed it as marking the accent. It seems to be more universal than people think.)

[LISTO]Marking the accented letter or syllable with a different style (italic, underlined, bold or in a different colour): Oĝalan, Oĝalan, Oĝalan.[/list]

(The bold might work for pronunciation guides, but personally I think that the acute system works best for non-loanwords, that is, when one is showing special stress on a certain syllable or word. I think that it’s possibly better than italics for this, since then one can set an entire block of text in italics.)

I think italicising the stressed vowel is good for dictionaries, accents are good for personal notes (as they same sometimes be confused with vowel length markers like in West Slavic languages), and the final apostrophe is good if it has final stress - it makes sense because what do you do in poetry or Esperant'? Your doing the same thing here, just treating the word the same as if it was a shortened Esperanto (Esperant') word