Only experience was years ago...when solar was new...we just tapped into busbars with any lugs that fit.....but it won't past muster now.
My experience is when i call a manufacturer of equipment they allow no taps...and we are required to not violate the mfctr product listing.
I now often recommend a new service, in such cases. Sounds like they may need one anyways for other reasons. (no room, no breaker space, low amps)
Not cheap, but can apply 30% tax credit.
OPTION 2
That said...why not replace 100amp main breaker with a 90A one, leave the 100amp breaker with customer
, and you are golden with a simple $10, 2p30A breaker for PV.
(located opposite feed on busbar of course)
(90+30=120A = 120% of 100A panel rating)