Here is an example build of what I described:
-Full Pyro tree abilities including top ability Thermal Detonator
-Viable off-heals (reduced Pushback so you will be able to heal somewhat while taking fire)
-Sacrifices 6% Aim increase due to loss of skill boxes
-Sacrifices Power Shot abilities (Power Shot is only used in extended fights when you are not being focused)
This build has the skill box that reduces pushback for heals. This is important when trying to heal or offheal in a flashpoint. And also there are a few skills that are swappable based on personal preference, such as having a better Vent Heat, or more Crit Chance from the 1st tier of bodyguard skills. (Also, this build is tailored for PvP and so there may be a couple boxes in the Pyro tree that can be swapped around as well. Such as Energy Rebounder and Degauss)
Here is a similar build, the one I use for PvP. It doesn't have the reduced pushback for heals, but it does have the reduced casting time for heals. The difference in this build and the one above is that this one will put out slightly more DPS because you can use Power Shots more (because you take those skills instead of more bodyguard skills.)
The disadvantage of this build is that if someone is attacking you while you try to heal, you get bad Pushback which slows down your heals and can make them un-useable. (But when left alone, you heal like a champ.)
Like I said before, both of these builds have several skill boxes that can be swapped around. This build is not as picky as other classes such as Assassin who have very specific Tank/DPS hybrid builds. These builds should be altered based on your play style. Definitely remove Degauss and probably Energy Rebounder if you dont PvP.
I queue every day as a Healer role in the group finder, in the 2nd listed full Pyro build with only a decreased healing time for my heals. Most groups never even figure out that I'm not a healer. However I wouldn't recommend this until your gear is very solid. I have full Black Hole, but the success depends on the group. Bad group, bad results. Good group, and no one even knows I have been throwing thermal detonators the whole FP.....