Just a quick update (unrelated to the Routertech firmware).
I tested the Draytek thingy (finally found a suitable power brick for it) but while seemingly a very good little piece of hardware it didn't fit the (changed in the meantime) requirements that involve using two separate VLAN's.
So after some troubles I went to a more traditional "PC-like" setup, using another piece of hardware lying around (a Fujitsu-Siemens Futro S300) with Zeroshell.
It turned out to be exactly the "right" thing for the chore, so I bought a couple of them (actually S220 instead) to have a "main" and an identical "backup" machine, and after some troubles with installing the Zeroshell all seems fine and dandy.
But I have not yet finished with the Mediacom's, most probably in the next few months I will have some spare time to dedicate to them, and as soon as I will have some news I will post them.