I have had PPP powder coat and print a bunch of enclosures for me, but none are from BYOC. I don't think that they would have any problem with you sending your own in to be powder coated and printed, but you would need to check with them first. To have them drill the enclosures requires very precise measurements. I honestly think it would be easier to send them the enclosures if they will let you. There is a fairly steep learning curve with making printable graphics if you are not well versed in that sort of thing. But I did write a tutorial on how I accomplish this, using a free vector editing program called, Inkscape. Send me a message if you would like a copy.
I painted, made water-slide decals, and clear coated all of my BYOC projects. I don't have plans to do anything but UV printing anymore. The results are production quality and durabilty, without the danger of a stray gust of wind ruining the finish while painting/clear-coating in my garage.