It certainly is possible to do this with Trill Flex.
We have experimented with sensing through different materials and surface thickness and have found that it is possible to sense through materials of up to around 2mm thick with the correct sensitivity settings. With a prescaler setting of 3 it is possible to detect finger position using a Trill Flex sensor mounted underneath a cutting matt for example. Bear in mind that the surface also acts like a low pass filter so the overall magnitude and positional accuracy is less than normal with touches being spread across adjacent pads.
Depending on your use case this may or may not be important.
For more information on changing the sensitivity settings see here: https://learn.bela.io/using-trill/settings-and-sensitivity/#customising-the-prescaler-setting
There is also some more info about sensing through surfaces and mounting here: https://learn.bela.io/using-trill/mounting-trill/
