It's generally not desirable, but there's no firm policy that blocks must have a single script value (and in fact, no such restriction exists in existing blocks).
If strong technical reasons exist for placing a character into the BMP, there will be temptation to fill a "hole" if the BMP is otherwise full. Likewise, many, many years (decades) from now, similar pressure might exist should the rest of the code space become filled.
I just noticed that filling in holes with characters that don't conceptually fit into the block would necessitate a different presentation of the blocks (by "presentation" I mean how the code charts are presented). Currently blocks have descriptive names. If the descriptions no longer fit, things would become messy.
Stephan
