I hadn't tried just using the .deb file but I tried this morning and it tries to install then returns "Unrecognized file. Failed!"
The installation is basically copying the binaries directory to /ffp/lib/plexmediaserver/, and creating some environment. You already have the environment, so you can install by just copying that contents.
I guess we're stuck on the kernel version until Zyxel upgrade?
Don't hold your breath. With exception of the 325 the whole NSA line is EOL, and won't get any upgrades anymore. For the 325 a last upgrade is planned, but I doubt if that will have a new kernel.
BTW, if you want a new kernel, then you can install Debian on your box. A suitable kernel can be found here
I have been looking at the Plex download page, and it's not clear to me if there is any ARMv5 package left. It's not specified anywhere. Maybe the 'Netgear Arm 6' in contrast with 'Netgear Arm 6 RN2xx'. The latter is ARMv7, so the first one might be ARMv5. (It's not ARMv6, there are hardly any ARMv6 SoC's around. The only one I know is the Raspberry PI v1. I think the 6 means dedicated for Netgear firmware v6).
For the Synology there is no ARMv5 anymore.
BTW, the 'Netgear Arm 6' segfaults on my NAS520, and when I force it to use the Entware libraries it doesn't run due to missing imports.
It would be nice if Plex could provide an Entware-ng package. Anybody on speaking terms with them?