I've been looking at those places to put the oil reservoir for the ScottOiler. In the end the reservoir ended up on the right hand side of the bike under the fairing, more or less the mirror image of where the R/R is located.
As long as the stuff you put in there doesn't interfere with the handlebar movement, and doesn't block the airflow around the radiator, you can use these hidy-holes for anything.
However, if these wires carry a lot of current (like wires for heated gear), then you need to know that you lose quite some volts across the length of these wires. For best results I would keep wires like that as short as practically possible. Same for a voltmeter: If you added one to your system, best results are obtained if the volt meter connects to the battery through dedicated wiring - not shared with any other electric consumers - and with the shortest wire run possible.