Did JVC or someone else ever make a better viewfinder for the GY-HD100u?

Not to my knowledge, having looked into the matter a while back.

Use a small HD monitor instead - my advice. Less costly, too.

No asked the same before I sold mine.
After I bought a HM700 I found it so hard to use the HD100 as the viewfinder and LCD on the HM were so much better so I asked JVC if there was anyway to add a new viewfinder or adapt one from the HM but the answer was no. If you are static a monitor would be better but if you run & gun then I guess your stuck with it.