How to proxy arp on tiny firmware?

Hello there,
my network is set up like this:
Internet --(DSL)-- FritzBox 7490 --(WiFi)-- OpenWRT --(Ethernet)-- Synology DS413

I am using relayd on the OpenWRT, and that works pretty well, except that ARP requests from the FritzBox keep the Synology from hibernating. FritzBox does not let me set static ARP adresses. So, proxy ARP to the rescue, which also works fine: ARP requests from the FritzBox do not (seem to) reach the NAS any more. However, now it sees ARP requests from OpenWRT. Well, I thought, static ARP on OpenWRT should fix that, but I cannot get it to work. arp -s does nothing (well-known, as arp is an alias for some cat ...). ip neigh does not support add. ip-tiny cannot be installed because I am missing some 40kb of space.

What other options could there be to solve my issue? I see two ways:

  1. Getting static ARP to work without arp -s or ip neigh add.
  2. Making proxy ARP simply not forward requests to the Synology NAS.

I know that I could get a VM, use the image builder to build firmware with usb-storage support, re-flash. use extroot to install ip-tiny etc, but I'd like to avoid all that if possible.

I'll be happy about any other idea. Thanks!