This, like all StackExchange sites, works in a Q&A format. Therefore, simply posting what you have done as a question would be frowned upon.
So, I would suggest that you post self-answered questions. Something along the lines of "How can I get my game to work on a ZX80?", wait a short time and post the answer.
Such self-answered questions are acceptable and would definitely be useful for other members of the community.
So, yes, sharing the knowledge that you have is not just acceptable but is welcome. It is just the manner in which you put it.
As to the tags, there isn't a ZX80 tag because nobody has created it yet. We don't create tags before there is a question to put them on. Users can create new tags once they have 150 reputation (on a beta site), but you can always ask the question and request here on Meta that someone create a tag for you.