Oh, that sounds so fucking hot... Yes, if there were panties underneath the gusset, it would be harder to get to her pussy. Normally I love holding my babygirl's panties to the side while I pound into her, but under a torsolette would be challenging.
I bought my mom the same Rago torsolette in a black, long leg version. She is visible through it and doesn't wear panties so I can use her through the split gusset!
Meine Mutter trug früher ab und zu ein solches einteiliges mieder (in weiss und schwarz). Mit richtigen Strümpfen. Das war zwar etwas altmodisch, aber es sah sehr elegant aus. Leider war sie nicht gefesselt...