There is a quote in the car racing community: “Speed costs money. How fast do you want to go?”
That same concept applies to your question:
I’m trying to print a little larger than recommended on the Taz5. Specifically a circle with a diameter of 285mm. The X axis is obviously fine, but I was hoping to stretch the printablilty of the Y axis by 10mm. Is this possible?
As you’ve noted, the Lulzbot spec for the build volume is 290mm x 275mm x 250mm. Since the build plate is 300mm by 300mm you can theoretically increase the build size by making a combination of HW and FW changes.
The first thing to be aware of is the fact that the bed heater is not 300mm by 300mm. If I remember correctly, it is 290mm by 290mm with a cool spot where the wires connect. If you want to use the full 300mm by 300mm area you will probably want to replace the glass/PEI plate with an aluminum plate.
The next issue is the physical HW. Probably the easiest way to increase the Y (or X) travel is to replace the Y (or X) rods and extruded chassis tubes with longer units. To increase the Y to 300mm you will need to replace the stock 500mm rods and extruded tubes with 525 rods and extruded tubes. You will then need to modify the FW to handle the extra length.