Front numberboards on Rivarossi Big Boys?

I guess that you mean to ask if the numbers for the numberboards are in a baggie? No. Numbers on number boards are generally put on for the train that the loco is running for most railroads. I don't know what the UP did for numbered trains but I suspect that the numberboards did have the train number there. Most modelers put the loco number there as many trains ran that way. I'll also note that adding numbers in there at the factory is a bit more difficult than with lettering elsewhere on the loco and usually isn't done.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!

So, if I owned a Big Boy and wanted the loco number there, how would I go about it? Decals or dry transfers? Is there a better way?

The UP was one of the few railroads which put train numbers on the locomotive numberboards. Usually the lcomotive number went there (I think Santa Fe was another that did it like the UP). However, prototypically, you could put the locomotive number on the numberboard, preceeded by an "X" - X4017, X4019, etc. That's how the UP did it when the locomotive was hauling a non-numbered or unscheduled train - an "Extra" - and still do, on 3985 and 8444.

But how would most modelers do it - decals - dry transfers?

I'd do decals as dry transfers will be rather difficult to get down in those holes.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!

Good point! Now, I have seen numberboards for Big Boys that were white letters on a black background. Did they ever do black letters on a white background? I have black decals - that's why I'm asking. ;-)

Frank, Bob is right - use decals. As to the colors - ALL Big Boys (and all other UP steam) used white numbers on a black background. Actually they are metal stencils painted black, over a translucent glass lens. This makes it easy to change numbers when they change trains.