What you can do is run that mat everywhere but only heat the parts that you want.
Using nuheat cable with ditra heat mat.
Uncoupling waterproofing vapor management and support to ensure a long lasting installation.
I liked the nuheat system better than the ditra heat cables so i bought the.
The nuheat cable may also be installed in combination with the nuheat mat system.
Schluter ditra heat integrates customizable comfortable electric floor warming with the functions associated with ditra.
Ditra heat combines the flexibility of loose heating cables with the ease of installation of mat systems.
Uncoupling waterproofing vapor management and support to ensure a long lasting installation.
Use standard mats for rectangular or square areas and fill in with cables for the irregular angled or curved.
Like you i wanted to use a decoupling membrane plus i like the waterproofing of ditra in the bathroom.
Ditra heat mat is about 1 4 inch thick.
Alternatively you could use ditra heat mat only where you want to heat but i would recommend another similar height tile underlayment to use with it.
Schluter ditra heat integrates customizable comfortable electric floor warming with the functions associated with ditra.
Ditra does not recommend using anything but their own heating cables but nuheat advertises that their cable system works just fine with the ditra heat membrane.
How to install nuheat heat matt with schluter ditra on a plywood subfloor.