Possibly it has something to do with how the ROM allocates memory for video RAM; could be a related bug in BII, or just that the original hardware couldn't handle that.
The ROMs supported by Basilisk II always expect the allocation to be in hardware (VRAM). (Only the earliest ROMs allocated this memory in RAM, considering the already high hardware costs at the time.) I'm not sure if any of the 32-bit ROMs had any leftover code to manually allocate the buffer. If there is, this could explain whole black screen problem: bypassing B2's allocation, drawing to a redundant buffer not seen by the emulator.
Basilisk II/SheepShaver bypass the ROM to allocate the video memory itself, and really all in one large block: RAM, ROM, VRAM. The emulator may face a few unhandled problems when attempting to resize the buffer; it easily crashes on my Windows system during restart. So if, say, the native ROM code tries to refresh the video base to the expected hardware VRAM, the emulator would be reading from high addresses that don't exist; B2 will crash if illegal reads aren't handled. (Build 142 traps illegal reads, and would run slower— one solid “blank” color, presumably white in 256 colors or lower.)
The technical limits for video are much higher than 1440x900, so that's not really an issue (besides ROM video drivers supposedly throwing a fit, trying to match the resolutions or whatever). For Millions (32bpp), the maximum width is 4,095 due to QuickDraw's use of the row size number to store two flag bits, effectively clipping the maximum size of a row to 16383 bytes.
High resolutions in earlier days were expected to be done in page/portrait format— larger vertical resolutions, useful for page editing/printing jobs. Some of the other limits, such as Handle size (8MB total) only apply to offscreen buffering not onscreen buffering.
The ROM video drivers of course never expected the high resolutions supported by the iMac. Very few video cards for 68K releases even supported Millions of colors. The Performa 636's (the one I have) video card with 1MB of VRAM would never leave 640x480 no matter the monitor connected, and never supported Millions.