The following code checks the bounds of each Winternitz digit:

This full check doesn't seem to be required. If the digit is negative, then OP_PICK will fail.

Therefore the following code could be replaced:

OP_DUP
 0
 { D + 1 }
 OP_WITHIN 
 OP_VERIFY

by

 OP_DUP
 { D + 1 }
 OP_LESSTHAN
 OP_VERIFY

If we don't care about digit uniqueness, we can reduce the size further and just limit it to D:

 { D }
 OP_MIN

3 opcodes are saved.
Am I missing something?