Thank you all for your help & suggestions!
I would give them a sample print and let them decide if it looks ok.
Working on this, though there is a complex combination of client, architect and the firm designing the construction of the light boxes (details of brightness and colour temp TBC)
I love it when architects play sign people...and to date, Ive never seen them do a good job of it. I will assume the the faces can be removable? If so I would suggest prints on clear think duratrans that would slide in on top of existing faces that way they could be changed out more easily. Digital prints on clear decals would also be good but that is something that would be better printed on clear poly....in my opinion
Faces are removable, held in with 6 bolts. I'd assumed that there would be a backing diffusion layer that prints could be sandwiched against. Will see if that is an option. Then either print on backlit front print film, or clear (I get a much better inkload on the back print, than on the acetate).
Starting to wonder if I can cut/drill/punch holes in the backlit front print film, and use those to secure the prints between the frame of the light box and the acrylic. Would require all 7 boxes to be manufactured to almost exactly the same size...
Test it first. Used to do some retail displays like that for one chain of stores so they could swap out promotions, but these went on the front of a lighted panel. Once we found the right combination of materials it worked OK, I didn't like it much, but they liked them. Rather than static cling, if they're going to swap & re-use graphics, look into GlassAppeal. Lays & stays better, and bubbles less than static cling. Best option would be like visual800 suggested, something on the order of clear poly.
Light boxes will be in a public concourse, front application definitely not an option. I can't print to glass appeal or cling, but I can see how this being used second face might make things simpler.
Print on clear, lam with white backed vinyl, apply in reverse on clear using rapid tac or a similar product.
The result is your print, behind a clear acrylic, backed in white (your diffuser).
Makes for a nice presentable product.
I hope this helps.
Trying this now - initial looks are showing OK density of print. the clear I have available is not self adhesive, so not sure this will stay put.
The work on display is set to change every 3-4 months, so this will be an ongoing production. Client is local, so I may be able to take front faces into our space, to apply print, if we do go down the slightly/semi adhesive route.