Yeah, there's still a few of us around here - you're not alone! We're running an older version of OpenACS so often there's not much need for me to post questions here, but I still read every day. Recently there haven't been many emails coming from OpenACS.org, but maybe that was related to a problem with the server?
Anyway, I think I found the cause of the problem (at least for my case, maybe not for your case). We have kernel parameter RegisterRestrictEntireServerToRegisteredUsersFilters set to 1. In ad_restrict_entire_server_to_registered_users there is this line:
ad_returnredirect "[subsite::get_element -element url]register/?return_url=[ns_urlencode [ad_conn url]?[ad_conn query]]"
As you can see it creates a return_url of [ad_conn url]?[ad_conn query], which won't work with a POST. Maybe it could be changed to use something like this https://openacs.org/api-doc/proc-view?proc=ad_return_url&source_p=1 ?
I'll investigate some more.
Brian