My guess is that battery could cause some "torquing" stress on your systemboard battery connections, I don't know if the battery support was ever designed to support the full weight of the laptop. With either a 6 or 9 cell battery installed, the unit sits flat on it's four feet, with this, the units weight is on the battery, probably 70% or more of the weight, and I'm not sure how much of that could be transferring into the system board, so I'd say use at your own risk.
That's my concern as well.
Maybe consider using something to set the laptop on so the battery hangs off the back, putting it back on it's proper footing.
If you're near a RadioShack, look for Gigaware "Flip-Up Feet for Laptops". They're being clearanced out, but would fit this job nicely if you can find them.
What would be nice is a power supply with build-in battery, it could work like a UPS.
Unfortunately, the power cable wouldn't support all the functionality that the current battery pins offer. Such an integrated transformer/battery would probably have to appear as just an adapter to the battery monitor and power manager. Also, bundling two components with differing failure rates/lifespans is not so good; just remember TV/VCR combos.
Along this line of thought, a slice battery would be best. It could accommodate the cells and still offer an incline. Of course, the drawback would be impairment of heat dissipation from the base. I suppose a cheap fan could be integrated, but there we go bundling again.