The main problem with this would be making sure the entirety of the portable hole doesn't go through the ring, so it can be pulled back. I don't know if portable holes can be opened while crossing an intra-plane portal, but tossing one through shouldn't be a problem. It would be expensive to carry around multiple portable holes and toss one through each time it fills up, but since you could retrieve them between dungeon crawls it would be a one-time expense, unlike the spell slots spent on Teleporting to and from your fortress (multiplied by however many hundreds of miles away it is). If you're tossing them through inter-planar portals, that would probably have the same issue as putting a portable hole into a bag of holding, but I don't see a problem with putting a permanent inter-planar portal inside of a portable hole.