What I mean is why is it not a good idea to use say, a Canon dedicated flash on a Nikon, or my Olympus flash on my Pentax??
My pentax has an external sync, is it possible to use that with my Olympus dedicated flash?
It may not work as TTL, but if I use a flash meter would it not work?

The manuals say not to, as it will damage the camera(s).
I am too afraid to prove them wrong for fear I would in fact damage my camera.
Is there a safer solution, or do I have to spend another cashload for a Pentax flash?
I hardly use my Olympus flash for all that matters. Only for fashion shows.
And yes, it was pretty costly. Ugh !

It depends. I use Metz for all my cameras. Some of the Metz flashes have replaceable electronics and shoe (SCA in Metz terminology) which is designed for each camera brand, so when I move from Olympus to Nikon and back, I change the SCA. Another reason why I use Metz, is that they have the "potato masher" style flashes with much higher output than conventional flashes (70 or above), which also makes a practical grip next to the camera. I've never liked the flash towering above my camera, and I've broken the shoe off a couple of them.

Metz is probably the best third party supplier of flashes and very reputable. Their models aren't cheaper than corresponding Olympus or Pentax flashes, but if you use more than one camera system, it may be worth a look: http://www.metz.de/en/photo-electronics/mecablitz-models.html

Click on SCA flash units. I use a model called 70 MZ-5. The new model is called 76 MZ-5, and is better and more powerful.