Not a total gimmick, but still not that useful. Stuff like the 800x600x60hz isn't actually displaying a 800x600 framebuffer, but a 540x600 frame buffer.Well there were some resolution dc supported if you messed with the way it did it sync signals and probably write directly to framebuffer. I remember a coder even created some 75,120,240 hz mode. If they were useful I am guessing people would have used them otherwise Its probably just a gimmick to say look my monitor says the Dreamcast is doing 1440x900.
Resolutions they hacked out of the dc:
The rate at which the DC's RAMDAC outputs pixels has exactly two speeds, one for interlaced NTSC/PAL/RGB, and one for 640x480x60hz VGA. So you can't make it output pixels faster for higher resolutions, but you can trade horizontal resolution for vertical resolution while keeping refresh rate the same, and you can trade refresh rate for total resolution. I've done 800x600x50hz and 1024x768x43hz, with full size framebuffers. Only one monitor I had, my smaller CRT, supported that last one.
You can also trade the other way, and reduce resolution for higher refresh rate. When I was adding VGA support to KoF, at one point I had the registers wrong, and used VGA pixel clock with a mix of VGA and NTSC pixel timings. The end result was KoF running double speed at 120 FPS (the reason the display is stretched vertically in the link is because of the mix of VGA and NTSC, it's fixable). Both 640x240x120hz and 320x480x120hz work; the first one has scanlines while the second one doesn't, at the cost of horizontal blurriness. Theoretically 320x240x240hz would work, but I don't have a display that supports it.
The official drivers for the DC never supported any resolution besides 640x480, 320x480, 640x240, and 320x240. The Sega's video initialization function takes bitmask to select the display mode, and one bit controls horizontal res, and another bit control vertical res. Earlier versions of the driver only support 640x480 on VGA; it wasn't until later in the DC's life that Sega added support for lower resolutions on VGA, which is why early low res games lacked VGA support.
The only "normal" alternate display modes the DC can do well (besides halving horizontal and/or vertical res) are 640x350x70hz and 640x400x70hz, which would be handy for stuff like ScummVM. It can't actually get the sync polarity right on those, but that hasn't really been a problem on my displays.