I have an IBM ThinkCentre 8187KUR desktop system. It has a bay for 5.25-inch floppies but no support in the BIOS. My question is, can I put in a PCI card to support floppy drive access and run the 5.25-inch floppy drive on this system, theoretically bypassing the BIOS?
One hurdle in your way is finding a suitable PCI card.
In the early days of PCs, most interfaces such as serial ports, hard drive controllers and floppy disk controllers were on separate ISA expansion cards. In the 386 and 486 era, miniaturisation meant that many of these functions could be provided on a single expansion card, commonly known as a Super-IO card. Manufacturers of SCSI interface cards, such as Adaptec, included floppy controllers on some of their cards, such as the AHA-1542. This meant that the user didn't need a separate card for their floppy drives.
By the times of the PCI bus, most motherboard manufacturers had started to include a Super-IO chip on the motherboard itself. There was thus no demand for PCI Super-IO expansion cards, and there would likewise be no need for SCSI interface manufacturers to include a duplicate floppy interface on their cards.
I've spent a few years looking for a PCI-based floppy solution, and have come to the conclusion that no such card was ever manufactured.
Alternative solutions include using a USB floppy controller such as the FC5025, which can read (but not write) 5.25" floppies, or developing your own solution, combining a PCI-ISA bridge with an old ISA floppy controller card. Or getting an older motherboard that still supports 5.25" drives.