OK this is where i hate to throw the wrench into your gears.
You stated that the player has RS232 control. this is not going to work with a USB to serial adapter.
serial is not necessarily RS-232.
RS-232 is a connection type that uses serial data communications. RS-232 is a +- 12V communications.
A usb to serial adapter is a TTL connection type. what this means.. a USB port only has +- 5 volts available to it. you cannot make 12 volts from the 5 volts. and 5 volts is below the threshold that RS-232 is capable of reading. PLUS if your RS-232 transmits to a connected TTV serial device it is going to cook it. because the TTL serial has a max input of +5v now there is a little wiggle room with that number but not up to 12v. now you can create a voltage divider using a couple of resistors that will bring that 12v down to a safe level that the TTL serial device would be OK with. but that still does not solve the issue of bumping the 6 volt output of the TTL device to a level that the RS-232 can read. so at best you can receive from the player but not send to it.
Now as far as the Mitsubishi TV is concerned if it to has an RS-232 connection it is going to be the same deal. i think that Mitsubishi TV's if you send the IR codes into the serial it will work. on some TV's they have an EX-Link port (Samsung) or something equivalent. which would be in the form of a headphone jack. I am almost certain you can buy a male to male mono headphone cord and plug directly from a blaster right into that port and it will work.
If the RS232 connection on either the TV or the player is in the form of a DB-9 connector. (old school serial mouse) it is going to be RS-232 +-12V if it is in the form of a headphone jack. then it is a crap shoot and either locate the specifications for the port or you are going to have to take a volt ohm meter and check the voltage on the RX and TX lines when doing something on the device.
Now as far as the ports are concerned. you may need to locate how to get into the hotel menu or the hidden system settings menu. sometimes the ports are disabled and need to be enabled via this menu.
I would suggest IP control because of the complexities with the serial on a device. so long as there are no limitations imposed by the IP control (as with Samsung and not being able to turn the device on).
let me see if i can dig up some information on both the specifications for the player and the TV as well.
I will need the model number of the TV
also i do not think this link is going to work