The new forum should have a field for a Bitcoin address, this field should check if the address is valid, also for security changing that address should take at least 7 days (avoid fast account hacking), also when the last update date should be shown when displaying the address.
That way you can tip any user without asking for any address.
I really like the idea of displaying the address update date, but not the 7-day security measure for the same reason raised by hilariousandco.
Might as well automate it to display the next address in the deterministic sequence once a tip is received. That way we can prevent address reuse.
It will be a good feature too, though some may prefer to use one single address to collect all tips for easier checking.