Since they will be used in lieu of license plates I just assumed they needed to be visible from the outside.
Referring to the Texas FMV LAWS thread linked in my signature block below, you are correct, the painted numbers need to be visible from the outside.
In service Military Vehicles with the numbers painted on the insides of the doors ARE VISIBLE FROM THE OUTSIDE when positioned ABOVE the seat cushions viewable through the window from the other side of the vehicle. The TxDMV
DOES acknowledge this as a standard military position for the marking of the numbers. Although, you are bound to have some LEO in the side of the road who might drool over the opportunity to cite you for it. THAT is why, throughout the Texas FMV LAWS thread, I repeatedly encourage folks to PRINT and CARRY all the laws they might have to have in a 3-ring binder for the roadside education of those less familiar with the various graces written into the LAWS of this great State of Texas.
Remember, we are each an AMBASSADOR for this hobby and must be willing to help the uninformed become informed.