I have the same problem, and I agree that a "fixup" option in the ASA would be useful. However, I found that one solution was to use a special DHCP option 54 (Server Identifier) for the particular DHCP scope. This allows the server to masquerade behind the WAN address, and thus enables the DHCP client to communicate with the DHCP server via unicast. Hope this helps someone other than me.
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