Is that a GB Studio issue?
Why not use the STAT interrupt to switch between the VRAM blocks so you can have all 384 tiles available? If you need 64 tiles for your popup (why that many BTW?), that would still leave 320 for our image.
Viewing post in About This Art Jam and tile limitations.
GB studio reserves 64 tiles for the UI and rendering text to the overlay. There are modes built into GB Studio to overcome this limit (which utilize the method you describe), but for the nature of this Jam, which is about imposing limitations and getting people a "feel" for how GB Studio operates, we include this restriction.