I have a bunch of those rockwool cubes from when I attempted hydroponics. I'm not a fan of it because I can't reuse it afterwards and I believe it facilitates an environment for mold depending on how wet it stays. The material is also a bitannoying to remove from plants that root in pretty hard, however I suppose for most species of Mexican Pinguicula it should not be too difficult. Nice to see you experimenting.
Very interesting idea. The roots are so superficial usually so they shouldn't get stuck in the cube. Just stuck my little ones in some rock wool little cubes with a little MaxSea.
I have already used this cultivation method with P.gigantea, it works very well.
be careful with watering though, an environment that is too humid for the Mexican can induce rot, especially during their
dormancy
I build a rockwool hydroponic setup for pings a while ago.
It was doing great for some species but not for other ones.
I dismantled it because the algae became problematic .