Today, I tried it, and it doesn't work. It works everywhere, but the hardware, where it should be connected.
I had several ideas what to do. Either I hunt out some PCI graphic card and keyboard with DIN connector or null-modem cable to connect it to other PC by serial (rs232) port. First I did the latter. I have a friend, who lent me a null-modem cable without data flow control pins connected (just RXD, TXD, ground and other two I can't remember now). Cable was long only 25 cm (approx. 1 ft), so he lent me a cord extension set with male-female connectors. Nice. The first thing I want to know if wiring is OK. One windows station and one linux. I ran hyperterminal on Windows and send data from linux. Nothing came, weird. I investigated, that the cord extension set wires bad pins.
I needed to know if at least the null-modem cable is allright. I made loopback COM1-COM2, ran 2 hyperterminals and after 3 tries I understood, that I hadn't switched off flow control. Doesn't matter. It worked from that timestamp.
The problem was with router. Nothing appeared on the hyperterminal console, just x sign just after I switched the power on. Strange, maybe grub can't initialize itself – I am going to enable serial support in grub tomorrow to see what happens. I'll have to disassemble the router to get the disk out again (I had to do this to provide console= kernel parameters), write configuration file and assemble the router back.
For the time being I still have no PCI graphic card, so the former thought I had in the beginning won't be perhaps realized, although it may be desirable.