I would like to point out that "ano" not necessarily needs to be a member of a real organization.

for example:
samĉambrano - room mate
samlingvano - someone who speak the same language
samreligiano - someone who is a believer of the same religion
and so on

Thus, I would recommend using "inismano" (follower or member of "inism" ) instead of "inismulo" or something with "-isto".

The way I learned it, "isto" points to some form of professionality, like someone who does something as a job for example, or someone who has profound knowledge and expertise in some field.

"ulo" on the other hand might be too general. So "inismulo" is just a inismo-person which might not be clear enough.

kiel ajn, I might be totally wrong and English is not my native language.
salutojn al ĉiuj

For the record... despite this discussion, and the dozen or so other suggestions, does everyone quite accept that Feministo/istino(?) is the commonly accepted word?

Freminismo and feministo are immediately available to Esperanto under rule 15 (international words).

And a quick search with Kukolo shows the words in use.

Vestitor:For the record... despite this discussion, and the dozen or so other suggestions, does everyone quite accept that Feministo/istino(?) is the commonly accepted word?

Yes, these are the normative words, used by ordinary Esperantists and also by the Esperanto feminist groups. There's a group called the "Feminisma Esperanta Movado", for example.

It would be confusing to try to change it now, these words are clearly in use. But if people want to discuss theoretical possibilities, with the understanding that this vocabulary is basically already fixed in Esperanto, go at it!