After looking at the makefile more closely, I found I was able to fake it out by explicitly using make KERNEL_VER=2.6. It seems to compile fine, except for one warning.
Running sudo make install KERNEL_VER=2.6 after this doesn't result in anything unusual, and sudo modprobe dc7280 likewise issues no errors, so I'm guessing the warning at the compile stage isn't serious. I'd like a second opinion on that, though.
MODPOST 1 modules
WARNING: could not find /home/user/highpoint/dc7280-linux-src-v1.0/product/dc7280/linuxla/.build/.him_dc7280.o.cmd for /home/user/highpoint/dc7280-linux-src-v1.0/product/dc7280/linuxla/.build/him_dc7280.o
LD [M] /home/user/highpoint/dc7280-linux-src-v1.0/product/dc7280/linuxla/.build/dc7280.ko