Using NAT in a bridged setup?

Hi there,
I am trying to rewrite destination address for a packet which is originated in a LAN behind a CPE so that it is forwarded to the AP with the rewritten destination address.

The CPE is configured as station bridge; the entire network is bridged. i.e. the AP is also configured as AP bridge and I only have a single LAN spanning the two, or, in other words,
the LAN behind the CPE is the same LAN that every other client attaching to the AP sees.

I need to match on the content of the original packet’s payload, so I have used an L7 rule which seems to do the trick, as the NAT rule is matched; however, the packet which is forwarded to the AP by the CPE still contains the original destination address.

What am I doing wrong?

Thanks in advance,
Rick