I would suggest getting a set of 'off the shelf' cycle indicators and then cutting them up so that the indicators can be mounted at the extreme edges of your trailer and maybe even on stalks. That would solve the visibilty problem.
You can then use the indicators to back up your hand signal as you ride with the trailer and revert to hand signal only when you are without it.
As a driver 99% of the time I would say that if a driver sees a slow moving box in the road with lights then the assumption would be that it is a vehicle of some sort. Whether it is human, electric or oil powered wouldn't even be considered. However, lights and indicator lights would be noticed and accounted for even if it is then ignored by bad drivers. But bad drivers would ignore everything, the rest of us would notice and respond suitably.
Relying on road position only is stupid and ignorant and is a practice that is too common by all road users nowadays and results in accidents that could only cause injury to any cyclist involved.
If a driver sees your box move out from the left of the road there is no indication of the reason or the intention especially as you wouldn't have brake lights either. What is the driver supposed to do? Move out with you to avoid an obstruction? Move to the left so that other following road users can see that you are turning right? Stay behind and slow to a stop with you and then sit there not knowing if your chain has come off or you are about to turn or you have dropped something or you are waiting for the cat to cross the road?
I'd say fit lights and indicators and a brake light if you can to suppliment your hand signals and ignore the debate about whether or not it would help. Anything would be better then nothing at all.