Adding static IP to Mac address mapping to your firewalls dhcpd configuration is now supported on ASA version 9.13(1) and later.

You need to do the configuration on the command line since this feature is not yet available in the ASDM gui:

config t

dhcpd reserve-address aaaa.bbbb.cccc INSIDE

wr mem

It works great, I just needed to upgrade the firmware for ASA and ASDM.

Best, Mark