Yes, the commander is required both to turn the LC-V300 on AND to command it to operate, otherwise it has no way on its own to be manually operated as a standalone device.
This is why I had to cave in and ship a commander from the opposite corner of the pacific.
One thing I found out is that unless you are doing nothing but karaoke the commander is not really all that useful otherwise as it's programmed to handle LaserKaraoke discs and lacks most of the standard commands you expect from a laserdisc player, like pause and frame select.
The service manual however states this is not something hard-coded to the autochanger. Pioneer also had a service tool that let someone completely substitute the commander in a technical service perspective and it includes all the standard laserdisc commands. I have never seen the tool and don't know what its part number was but presumably if you crack its secrets suddenly you can make the LC-V300 be a 72 disc laserdisc player and the possibilities stem from there.
Mind you, a serial interface may also already exist as well since the LC-V300 was also available in an industrial package. The closest thing I can find is the Pioneer LC-V330 which on the outside is visually identical to the V300 aside from the laserkaraoke badging and front window removed, there are conflicting reports on how many discs it can hold and while I DO have the MS-DOS software that lets you control one over a serial port (you can find it
here), that I can tell it uses a different commander interface with the more standard 15 pin connector instead of the 25 pin port the V300 uses. There's still a lot of presumably Pioneer documented but not available online mysteries.
Edited: Pioneer lists that the service manual for the LC-V330 does exist but is not available for download -
http://parts.pioneerelectronics.com/...uctNum=ARP1666