It would be interesting to see if your light sport glider solution would work. I will have to research further.
You are not treating the light sport glider as an "add-on" rating. Is there solo requirements before the "check ride "?

I think it takes more than a log book endorsement for a rating, it takes an 8710-11(?) which would issue a sport pilot certificate, but that would be for someone with no pilot certificate. Yes, more research required.

For many years, many new soaring pilots have achieved their glider add-on from a "non-current" certificate. But now that some asked the question and got a "legal interpretation " now all DPEs are forwarned to check for a current flight review, that they never looked for before. Again the soaring community takes a hit.

BillT