Yep, you can Steve.
Even with the Tcon, the combination of a compact package, very fast (f2.8 through the zoom range) lens and image stabilisation do a great job.
The attached picture was from about 40 feet using the Tcon: "only" a swan, but the image quality is right up there, I think.
Some info, Doug...
You mention that an FZ20 + Tcon is getting perilously close to the bulk and weight of your dSLR + lens.
I've just done some checking and:
the Nikon D70 + Sigma 70-300 weighs 3lbs 1oz,
the FZ20 + Tcon + Pemaraal...
I use the Pemeraal PA55H adaptor: adaptor onto camera, Tcon onto adaptor.
I've not noticed any quantifiable image degredation Doug, though I've never done any on/off comparisons using my own camera, but I can say that with the Tcon in place, more light gets into the camera than...
Snappy, is it possible to attach the TCON17 teleconvertor (and the like) onto the FZ5?
My understanding that you couldn't, which was why I bought an FZ20 - the idea of 734mm, f2.8 and image stabilisation in such a small package is very appealing!
I just mention this...