How to programm a standalone MSP430g2553 via a com port?
The details of the MSP430 series boot strap loader (BSL) can be found here:

http://www.ti.com/lit/ug/slau319i/slau319i.pdf

You will need to connect RST and TEST lines to DTR and RTS as described.

Energia uses mspdebug to program the Launchpads using the rf2500 or tilib options. The flash-bsl option should be about the program the MSP430g2553 via the COM port with the RST and TEST lines connected.

In the Energia folder under hardware then msp430 there is a file called boards.txt. You should add a new entry for your board with the upload setting set to flash-bsl.

e.g.

##############################################################
baremsp430g2553.name=msp430g2553 (16MHz)
baremsp430g2553.upload.protocol=flash-bsl
baremsp430g2553.upload.maximum_size=16384
baremsp430g2553.build.mcu=msp430g2553
baremsp430g2553.build.f_cpu=16000000L
baremsp430g2553.build.core=msp430
baremsp430g2553.build.variant=bare
baremsp430g2553.upload.maximum_ram_size=512


Note: I have not done this myself, but it is how the Arduino system is modified. I'll be putting together a custom msp430fr5969 board in a few days and following the same process.

Of course, you do not need Energia to program the chips. You can make a normal msp430-gcc build system and use the Energia core libraries and compiler. I do this myself using Netbeans and it is well worth the effort to set up. The "Processing" IDE (what Arduino and Energia's IDE are based on) is great for a quick mash up but terrible for any serious work, in my opinion.