Aha - I knew this was discussed. Here's a link to Site Admin Kimberly C officially declaring that the site owners have deliberated, and interpreted the ToS in favor of allowing off-site links in signatures. So there's that.

http://www.renderosity.com/mod/forumpro/?thread_id=2883717&#msg4174493

"After talking further with the team we have decided, after receieving more intake from other members, that we will allow off site linking in sigs."

I have to assume that if you can spam such links in your sig, then you can write them in posts, too.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)