I'd like to use the r.catchment tool from GRASS within QGIS. It looks like this is a tool that hasn't yet been developed into a processing tool under the GRASS commands. Is there some other way to load GRASS tools within QGIS?
Yes, beside GRASS processing tools you can use GRASS 7 plugin (available from QGIS 2.12) which integrates GRASS Environment directly to QGIS. You have to activate it in menu
Plugins --> Manage and Install Plugins ..., then GRASS menu appears in
To use this approach you have to be familiar with GRASS and how it works with data (see QGIS documentation). For hydrology analysis you can also use a SAGA tools from processing toolbox.
To avoid some frustration:
I am afraid, the answer is you currently cannot use r.catchment out of the box from QGIS, because r.catchment is an AddOn and neither Processing nor the QGIS-GRASS-plugin allow for using AddOns.
The only possibility I know of would be, if you add that module to the QGIS-GRASS-plugin yourself. That means: copy the script to the plugin source 1, add a module description (.qgm-file) for it to 2 and activate it in the module tree 3. You can do that in your local installation, and do not have to compile anything. Just reload the modules when you are done...
If you need r.catchment, and esp. if you run hydrological analysis on a regular basis, I would simply dive into GRASS!
Please have a look at: