My bar is PPC black and has stood up to all weathers the only "problem with PPC is if it gets damaged then water can get under it and lift it off in a big sheet" to combat that I had my bar shot blasted and it left a very good key (too good as the powder coaters said they doubted the powder would cover it smoothly) I said not bothered if it resembled black painted sand paper as long as they coated it as evenly as they could I'd be happy!
Result it looks smooth and I know it's well keyed.
Running the cable along the bar, only way forwards would be allong the under edge of the bar you could place grommet holders at regular intervals this would give the bar an even more professional touch than say tie wraps or you could incoporate a grove allong the whole edge of the bar to take the cable with a clip mounted behind it at intervals this could be machined before and bending takes place (the latter harder to produce due to clearance left after powder coating) lastly tube of the correct internal diameter could be also incoprorated into the design of the bar (but that would be open to water ingress)! Hope one or more of those ideas will help!
Oh consider the mount for the 7pin socket in the design of the bar mine was a plate supplied for a Binks tow bar that was not used (I welded that to the bar before it was blasted and coated! Looks very neat!)
Trailer electrics are a lot simpler run the cable inside the draw bar (presuming you make this from hollow section to keep weight down) depending on the design of the body on the trailer you can go several ways after that, try not to make joins in the cables where water will be any kind of problem 7 core cable the most commonly used still needs 2 extra bits to run the opposite brake and Earth to lights. (Do not forget to have rear relectors on the trailer! Leagal Requirement) Side makers look smarte too but nor 100% sure the are needed! But anything that maked the trailer more conspicuas the better! Ref unsuspecting motorists!
Key to colours BLack = Left Rear Light
YeLlow = Left Indecator
GReen = Right Indecator
ORange = Right
Red = Stop Lights
White = Earth
Blue = Fog Light
These are the british STANDARD colours for what they do if you wire the bike up this way any commecialy sold trailer will work straight out the box and your mate can also pull the trailer with any car with electrics fitted to it! A buzzer must be wired in line with the indecators (supplied by most towing centres) these tell the driver that the trailer is still attached it only buzzes when the indecators are on so no buzz = no trailer lol.
When making your bar allow provision for a brake away chain (legal requirement!) to be attached to it (simpler to do while manufacturing!) You can no longer just loop it around the neck of the ball as this was commonly done! It is illegal!! As some chains/cables were worn through by the action of the hitch on the ball. Plus a chain will reduce the angle of lean available on a fixed hitch! As it gets between the hitch and shoulder of the ball which in turn reduces clearance!