Store your code under the ‘python module’ section on your Scripts tab (as opposed to the Tools/Script you have shown. If you have a method defined like this: def yourMethod(): print “This method is just like so sweet, you guys.”
Then you can access it later from outside of the HDA itself (like from a shelf button) with something like: hou.nodeType(“Sop/yourhda”).hdaModule().yourMethod()
Then to see what's going on inside of burn, you can do the following: add a Python Shell Pane and type: >>> import doppyrotoolutils >>> help (doppyrotoolutils)
That will show you the docstring for each method in doppyrotoolutils as well as the location of the .py source file.
You could take pieces of that code and add it to your HDA, or simply have the build script call doppyrotoolutils.burn(), but I think it might still do the object selection prompting. You'd probably need to tear into it a little for it to use another object.