Dynamically generated disassembly is not currently supported in the disassembly window. However, you can use a format specifier to disassemble a chunk of code:
addrress_expr, disasm [num_bytes_expr]
This specifier will disassemble 'num_bytes_expr' bytes of instructions starting at 'address_expr'. The 'address_expr' can be any expression that resolves to an address. The 'num_bytes_expr' expression is any expression that resolves to an integral value. It is optional and defaults to 128 bytes if not specified.
As Martins stated, when dynamic codegen is properly supported, you'll need to add a runtime function entry for unwinding / step-out to work as expected.