You will suffocate the tree roots I would think.

I know I would need to protect the trunk by not allowing the dirt up against it (with a collar (a box within a box) for example)) but I hadn't thought about the roots themselves - thanks

The maple roots will just invade it anyway.

Raising the soil 12 inches around the collar of the tree will slowly kill the tree. Even if you did that, the hostas would only improve for a short period of time. The roots would soon fill the area. You can try burying pots in the soil or purchase spin out bags. Just yesterday I pulled out a spin out bag with a hosta planted in it. One maple tree root pushed its way through the bag. That one root slowed down the growth of my hosta.

I have read a few different ideas online in the past:

One thing that I have read time and time again is that adding too much soil (more than 2-3inches) on top of tree roots will hurt the tree.

Try using double nursery pots appropriately sized for your hosta rootball - using 2pots doubled helps to turn the pot every so often to break off any invading tree roots.

Spinout bags planted inside out (so the copper chemical is exposed to tree roots to repel them good for a few years) - however I have also read about the bags failing at the seams with roots entering that way

Try not to disturb the maple roots- the more roots that are broken or cutoff the Maple tree will send out more roots in that area.

Plant Larger hostas closer to truck in between the large tree roots, smaller hostas further out (where a small hole should not overly disturb the mat of tree fine roots causing it to grow out more roots)

Water the entire under tree area not just where the hosta is planted so that the tree roots will not torpedo the hosta rootball area in search of the deliciously watered and tended soil.

Make a small hosta bed with thick pond liner (not weed barrier cloth) bring the pond liner up above soil. But this pond liner method needs to be thought out and made properly for hosta drainage and depth etc.

Easiest and most guaranteed method of root prevention is to use containers and pots above ground, placed around the tree canopy. Plant other perennials with smaller mature roots into ground or try spring blooming bulbs. For winter bring hosta containers into an unheated dark cold storage area or tip the container over once the soil is frozen onto its side (all winter).

Good Luck, hope you find a method that works for you.