I have used chopped leaves and grass clippings as mulch. Anything organic can serve as mulch and it will decompose some over the winter and then next spring. I say go for it.

I always use chopped leaves for a mulch on all my gardens. In the spring I just 'fluff' the leaves only removing the ones around the iris. All the others get left and the plants come up through the mulch. Helps keep the weeds down and then later feeds the worms. The leaves will 'disppear' by about July feeding your garden as well.

As for dropping pots in the ground for the winter it works. Although I have a tendancy to remove the commercial soil in the pot and replace it with something more sustaining such as triple mix with some more compost and vermiculite added. Sometimes the growers mix is meant as a temporary thing till they get planted. I wouldn't worry if you intend to plant them right away next spring.
I have hundreds of pots both heeled in and not. The not ones I place the larger pots on the outside and the smaller pots to the center. Surround the pots with bagged leaves, then shred leaves and spread them over top to about 6". In the spring I rake the leaves off the top. Yes I lose some but mostly the ones in smaller pots. Most are in 1 gallon size and up.

I must add that I am a zone 5b tending towards a 6 because I am sheltered in the city. The heeled in pots are in a 5b with harsh west winds.
It all depends on the winter.

Dana

I planted my last clematis two weeks ago, with mulch to keep the ground from freezing as quickly or doing the freeze/thaw cycle. I've also buried clematis pots in loose wood shavings & successfully overwintered them. I've had more difficulty with plants in pots in the ground because of standing water during the spring thaws - we have a 'mud season' during which the surface is quite wet, but lower frozen layers don't allow drainage.