Sorry to answer your form email publically, but you ask VALID questions that perhaps others have similar interests in. But, thank you for the email form as well, I have no issues with people hitting my web site up and asking me stuff via those means.
The DP-415 is meanted to be used as a truss or tree mounted dimmer pack. The advantage is you can run power to the pack, and then the pack provides power to nearby fixtures. It makes more logical sense with pictures.
Now, you can remove the back panel to flip the bracket around, thus making it even easier to truss or tree mount the dimmers. Of course, it's best done with the power DISCONNECTED from the dimmer. Remove the screws from the back, flip the back panel around, then replace the screws and there ya go. You may wish to do the same thing anyways, based on your concept.
You expressed you wanted to mount these on a board in a ventilated cabinet. It sounds like you're doing this for a centralized management issue for a permanent install. Do keep in mind that you'd need to then run LONG power cables from each dimmer channel to the fixture. Ideally, you shouldn't need to keep changing the addresses of your fixtures.
I'm not sure if the jacket on extension cables is rated or approved for in-wall installation. But that is trivial as you can have an electrician run proper wiring for you
Back to the cabinet/board mount. Obviously, the dimmer's electronics do NOT touch the back panel, so they are definately not using that back panel for heat sink applications. Would I mount them on a board? No, I don't think I would. The manual states it would like 6 inches of clearance, and placing them on a board would somewhat mess that up a bit on the clearance stuff. If I had to do this(and I wouldn't because I'd use something else if I wanted an install configuration that would end up costing a LOT more), I'd stay you can still build or us your cabinent. But I'd find 1.5" wooden dowels(1" is fine) for hanging the dimmers onto, then a measured distance for another dowel for the bottom 1/4 of the pack to rest upon. Then, continuing with your idea of ventilation, you'd be in find shape. Is this going to be an enclosed cabinet? Maybe a glass front? Of course the dowers would have to be mounted a bit from the back.
Really though, I've seen thousands of clubs with dimmer packs in their trusses for permanent installations. Some want the LED displays viewable, others hang it so the displays are hidden. Ideally, the best application is to have the dimmer out there near the lights, where it is intended to be. Really, you shouldn't need to change addresses once you've got lights attached to it.
By the way, true rack mounted dimmers tend to be rather expense on the per channel basis, so it could be cost prohibitive to go this route. You may up for it in them often lasting darn near forever and having the options you need(dim/switch and addressing) in the centralized location you want. You also get higher channel density over what you'd get from hanging dimmers in the same amount of space(usually).
What is your end application? Nobody wants to lead you down the wrong path.