graham
From the first script that is run you could execute a source command to manually source your other script.
It's a bit trickier than that, as you want to make sure that your environment is consistent, i.e. that all appropriate scripts get read, regardless of whether they are present. Ergo daisy chaining scripts doesn't work really well.
One solution is to write a wrapper to launch Houdini which effectively does:
houdini myFile.hip /path/to/setup.script
And then the setup.script looks at the HOUDINI_PATH, reverses the order, and then looks in each location for a 123/456 or some other named script. If it finds it, it runs it; repeat for all remaining locations in the HOUDINI_PATH.