- Is there something similar to Reddit's /r/freegold/ for bounties?
No. Individual sites may have a meta post or a chatroom dedicated to bounties; if you have a plan to regain reputation (see below), you could try posting your non-official bounty there.
- Am I allowed to post on a SE's meta to request this?
You can try, but I guess such a post will be met with down- and close votes. E.g. here on Meta we have a close reason "This question does not appear to seek input and discussion from the community."
Eg, my family and I are unskillful at computers and have many questions of obvious public interest and benefit at Super User, but I don't have enough reputation to keep placing bounties.
Super User has a lot of posts which can be made better by grammar/spelling corrections and formatting improvements. If you restrict to suggesting edits on relatively new posts, I guess they will happily accept your edits; a bounty costs only 25 approved edits, and since you can gain 1000 reputation from suggested edits, that's enough fuel for 20 bounties.