You definitely can, but be vigilant on monitoring RH, especially in a Tupperdor. The PG/water solution ideally produces 70% RH, but there's certainly the potential that it can go much higher. Still, if you have a cheap source of PG, it's cheaper than buying beads.
Really, those sponge-type humidifiers should always be filled with PG solution. Straight up water is practically begging for grossness.