I'm using OR cornice mittens with a removable primaloft liner. I'd like to have another liner that I can swap in when the primaloft ones get soaked. MEC used to sell one, but I don't see it anymore. I have the OR PL400 mittens, but they aren't enough.

OR uses something they call moonlite pile fleece in their northwall mittens. Something like that would be good. Primaloft would also work.

Well the Hestra Primaloft Extreme Liners are extremely warm, but are quite bulky, so they would need a very big shell if you are going to use them in a shell.
http://www.hestragloves.com/#/en/gloves/liners/primaloft_extreme_liner/

If I remember correctly, they weigh 160 g (for the pair).

I believe the palm is 400g Primaloft, and the back of the hand is 600g Primaloft, so that is 10x the insulation normally found in apparel. There's a lot of loft.

I think it was on this site that I read a recommendation for Dachstein mitts, and got a pair --- I haven't had them out in super cold weather yet, but my brief experience has been that they're too warm if anything, when other mittens are "okay". A bit on the heavy site, but worth a look.

Call MEC they had them this winter...
I think Goosefeet does a down mitten too or OR 400?

Ragged Mountain Tuckermitts

http://raggedmountain.com/index.php/hats-gloves-socks/mittens/tuckermitt.html

I have had Dachstein mittens since 1977 and they are great.

But my warmest liners are double layer fleece by EMS. (No longer made) But you can buy fleece at a fabric store and make them yourself. Just be shure your shells are large enough to accomodate them.

Or you could buy pile at the fabric store and use that for a single layer liner.

blackrockgear.com has 900 fill UL down mitts. if you didn't need insulation on your palm (for climbing or w/e) these would be an elite option.