Thanks to all who made suggestions. It is now clear that there's no direct, supported way to accomplish my original goal with this BIOS-only system.
I've decided to stop short of going through processes that require reading tens of pages of forum threads for specific instructions and downloads of different operating systems in order to set up a system that boots a single large partition. The difficulty in doing it and especially the risk of it stopping working in the future are too high.
At this point my alternative is to set up 8 x 512 GB SSDs as a 4 TB RAID array that I partition into a max-sized boot partition (C:) and a second partition containing all that's left (D:). I'll move from C: to D: several large sets of data (in the hundreds of gigabytes range) that grow slowly or not at all - my photo data, astronomy data, and active virtual machines.
This approach will 1) have all the drives working in tandem for all I/O operations, boosting performance a good bit over the 4 drive array I have now, and 2) there will be much more free space available for temporary use as needed on C:. Both partitions will have room to grow for quite a while.
Using the current 992 GB of C: drive data usage at the moment as a basis, this reorganization would initially work out as follows:
C: 275 GB used, 1800 GB free
D: 717 GB used, 1100 GB free
The only real downside is that the free space is not in one usable chunk, but two.
And while having two separate partitions would complicate my backup strategy slightly a little, in fact it could be somewhat an advantage to remove a large chunk of data from the regular System Image backup and rely on my file backups (which currently also include these files) for the data moved to the D: drive. I'll probably have to get a bigger second external drive (right now I have a 3 TB and a 1 TB USB drive; the latter would be replaced with a 3 TB or 4 TB drive to allow room for growth).
Thus I'll have plenty of room for both C: and D: partitions to grow, as well as improved I/O performance, for a long time, no doubt well beyond the time this workstation is obsolete (and then I'll just move the array to a new one).
It's nice that SSDs are so compact and use so little power that it's quite reasonable to put 8 of them inside a desktop chassis. But even if I couldn't, there's the possibility of an external chassis. Decent RAID controller cards like the Highpoint 452x series offer both internal and external capability.