Let's clarify the underlying problem.  It sounds to me like the problem is "I can't quickly find a particular tab".  Both the omnibox "switch to existing tab" feature and the upcoming attempt to try preview cards for tabs are aimed at making it easier to find tabs in the general case.

It could be that there's some sort of problem of "I have 700 tabs" here, in which case doing things that would help avoid having lots of open tabs or give better/easier tools to group them into manageable tasks (within which individual tabs can be found quickly) might help.

The solution proposed is difficult (e.g. to make work across custom/system themes) and only tackles the case where the reason you want to find the tab is that there is ongoing data entry.  This particular solution sounds like it would serve better as an extension.  But that doesn't mean the problem isn't worth solving.

yes the issue is "I can't quickly find a particular tab" - but the tab I am trying to find has pending data so should be relatively unique - i.e. is one of the (few) tabs that would trigger a 'are you sure you wish to navigate away' when closing Chrome.

I think tab groups might help here, as I could in theory group any tabs where I am actively doing work into a tab group, but given I often have many many tabs for a given work-task I'm not entirely convinced it would help.

re: themes, that is a good point, and probably makes this suggestion as-is unworkable. Perhaps another idea would be to make tabs in these state (there is pending data entry) easily searchable via the omnibox (when you don't know the title of the tab, just that there is a tab with pending data in it).

FWIW, in the meantime, I do have a temporary workaround - just try and close the entire window that contains the tab I know I have pending data in, and Chrome automagically switches to the tab... the solution isn't totally ideal though, because if I close the wrong window (the one not containing the tab I was just editing) then I lose a load of tabs :)