OK, I ran my experiment tonight using the new LMDE MATE RC. I used sudo pluma to remove preferences under apt, and to block Mint sources and set just wheezy sources. I added debian multimedia with the keyring, and the MATE desktop for debian source and keyring. I then ran updates in the terminal--which took a long while. I rebooted, and everything looks fine except the mintupdate manager is off, it wants to update and remove some Mint stuff, so I just removed it from the startup menu and will use the terrminal for updates. The following packages are being held back in updates:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
bind9-host dnsutils firmware-linux firmware-linux-nonfree hunspell-en-us
libbind9-80 libisccc80 libisccfg82 liblwres80 libqt4-dbus libqt4-declarative
libqt4-designer libqt4-help libqt4-network libqt4-script libqt4-scripttools
libqt4-sql libqt4-svg libqt4-test libqt4-xml libqt4-xmlpatterns libqtcore4
0 upgraded, 0 newly installed, 0 to remove and 24 not upgraded.
This worked better than I expected, but I think one would have to be careful with updates to make sure nothing important gets removed. In the past I always had issues with Debian wanting to remove mdm during updates, but so far so good this time. I think adding the MATE source probably makes all the difference with keeping this setup together. And probably someone who knows more than I do could set apt-pinning or something to further secure the setup, but like I said, so far so good.
edit/update: forgot to mention during the update a script stopped it at one point, and I entered the letter q to get it going again. I also entered the letter n to keep defaults during the update. Just to spruce up the fonts I grabbed an Ubuntu Natty fontconfig file. You really would not think this is a Debian install.
My LMDE MATE Wheezy:http://imgur.com/ST4dCwC