I did a week long retreat last year using a prototype tDCS device made specifically as a beta for helping mindfulness practice. I found it useful as an aid definitely in relaxation and as a byproduct for focusing. I would do it more often if they figured out a cheap, sustainable way to put the electrodes on the head without all the gels/sponges/etc.
I doubt that's going to happen anytime soon. ECG/EKG/EEG's require the probes to have a good electrical connection to your skin. And dry skin is a pretty terrible conductor.
The gels help the probe adhere to you skin and provide better conductivity. You'd still need something to hold it in place anyway.
Protip for removing the gel: Warm water helps a lot :D
If you ever have a sleep study for example they place them all over your head and it's a little messy, warm water lets it wash right out.