Run xvidtune and collect the data it shows. Here's what I got:
Vendor: SAMSUNG, Model: SYNCMASTER 240T(DIGITAL) Num hsync: 1, Num vsync: 1 hsync range 0: 29.00 - 81.00 vsync range 0: 39.00 - 85.00 HSyncStart: 2048 HSyncEnd: 2256 HTotal: 2592 VSyncStart: 1201 VSyncEnd: 1204 VTotal: 1242 Pixel clock: 193.15 MHz H-Sync: 74.52 kHz V-Sync: 60.00 Hz
Using these values and the comments in vgadb, I came up with the following entry:
sm240t=1920x1200 clock=193.15 shb=2048 ehb=2256 ht=2592 vrs=1201 vre=1204 vt=1242
Look at those numbers. You'll see that "clock" corresponds to the "pixel clock" value from xvidtune; shb, ehb and ht correspond to HSyncStart, HSyncEnd, and HTotal, respectively; vrs, vre and vt to VSyncStart, VSyncEnd, and VTotal.
Once I had this entry added to /lib/vgadb, all that remained was to edit plan9.ini to say "monitor=sm240t" and "vgasize=1920x1200x32". This method should be applicable to most monitors.