I wonder if anybody tried the workstation counterparts of the B150 and H170, namely and respectively the C232 and C236 chipset based boards (these are probably the exact same silicon but with microcode support enabled for some entry level E3V5 Xeon CPUs and ECC memory [when the Xeon or Pentium/Core CPU also supports ECC] on top of the "lesser" consumer CPU and memory parts).

These boards aren't exactly cheap in general (but neither too expensive compared to some of the "fancier" consumer parts, especially if they live up to their promise of being more stable and reliable than their consumer counterparts) and I am yet to see any SKU with >5 PCI-E slots (they might restrict the numbers on these boards for an inherent reason rather than wiring up countless slots regardless if they will work or not ) but they might worth a closer look and try (search for a relatively cheap one with >4 slots, =6 if any and give it a try...).