Did you buy the dwarf variety? If you did it will not get into the drains unless they are very close but if you didn't it would die of starvation as they want to reach thirty feet high and wide. Even with a dwarf, if you are going to bury the pot make sure its big enough as getting it up again to repot in a larger one won't be easy.
Tiny drain holes are never a good idea.

You'll need to keep pruning your tree to keep it dwarf, too. You're right about willows too close to the house foundations/drains, so be careful how close you plant it, in spite of your 'barrier' idea. I'm not convinced it'll work, either - you might lose the willow if its roots are too constricted for it to get nutrients.

Thank you for your replies Steragram & Spritzhenry. I'm pretty sure it's not a dwarf, it's a cutting from my mum's friend (who has passed so it'd be a lovely commemorative to be able to see it properly on my front lawn) and has grown quite vigorously in it's restrictive pot. I put it in a slim cylindrical metal pot initially while I decided what to do with it and it's done amazingly well considering! I think I'll repot it in fresh soil and plant food and half bury it with maybe some hostas around the perimeter and see how it gets on. I can always lift it if it seems to be struggling. Thank you very much for your advice which has helped me reach a decision! :-)

If its not a dwarf its not really a good idea to plant as you suggest.

Make sure it's WELL away from your house, won't you, and keep pruning it once it's established. Mine is less than half the size of another unpruned one in the village. I shape it every year.