I use it elsewhere, no water absorbtion.
Any reason not to do a full flush and fill on the bobber when I swap over to the GS rim?

DOT 5 doesn't absorb water, but water still gets in. The water doesn't go into solution in the fluid but it still collects in the calipers and master cylinders over time as little blobs of pure water and can cause corrosion.

Harley used it for a few years in Sportsters and it caused problems when the owners didn't change the fluid often enough. I've got a 2004 XL883 that uses it and the owners manual recommends flushing and replacing the fluid every 24 months. HD switched back to DOT 4 sometime in the mid 2000s.

DOT 5 also doesn't mix well with DOT 4 so if you don't get whatever fluid is in there all out, that might cause issues too.

The only advantage to DOT 5 that I'm aware of is that it has a higher boiling point than 4. That could be important in race bikes that get the brakes really hot but I doubt that that makes much difference in most street bikes.

My opinion, which is just that, is that unless you get the system perfectly clean, and change the fluid regularly, using DOT 5 in a system that was designed for, and works satisfactorily with DOT 4  probably isn't worth the trouble it might cause.

Interesting side note:

When I bought my Sportster I had no idea what kind of maintenance the previous owner did so I immediately changed all the fluids. DOT 5 brake fluid is purple when it comes out of the bottle. The fluid that was in the master cylinders and that came out of the calipers when I started pumping it out was yellow. I thought "crap this guy put the wrong fluid in the bike. I went ahead and flushed and refilled the brakes with the DOT 5 and hoped for the best.

I did some internet research and discovered that DOT 5 fluid actually does loose the purple color over time. I haven't had any brake problems yet, so I assume the fluid that I took out was DOT 5, just old. I'd never dealt with DOT 5 before and I wasn't aware of that particular characteristic.  

Short answer, not a great idea.
Long, boring answer with personal experience and details if you insist.

Back in the 70's when I believe the DOT 5 fluid came out - it was supposed to be the "go to" fluid for collector cars that sat for long periods of time.

I really don't know why - but it never really caught on and became a popular replacement brake fluid.