It may depend a little on what your driveline angle is but more than likely it will leak out the back if the slip yoke is not installed.  You can purchase a low-cost pump that will screw into lube bottles or that you can put into the container and fill the transmission from the side if you have adequate access to the fill plug.  Most of the chain autoparts stores carry the pumps and although a little awkward under the car they work pretty well.  A funnel with a long hose works pretty well from the top if you have a helper.  Or, get your driveshaft made up, install it, and then fill from the top.  If it is an access issue with the shifter, you will probably have enough clearance with the slip yoke to drop the back of the transmission down to get the shifter off to fill it that way.  I think you may find that filling through the actual fill hole is not as difficult as you think.  Is this a used T-5?  My transmission came with a rubber plug that slips on the output shaft and contacts the seal.  It worked surprisingly well.  To install the driveshaft, just pull the plug and insert the driveshaft with minimal leakage.  Last, on an already too long response - if you install the slip yoke you can fill it to the correct level.  Put the back wheels up on ramps after filling and the angle will keep the oil in the trans and you can pull the yoke.  You'll need it up in the air to install the driveshaft anyway.

Thanks for all the info on the T5 fluid and greasing the tailshaft seal.   I made some helper guide studs to stab the transmission in.  I ended up pulling them out.   The T5 went in better without them.  Seemed like moving the hole transmission back and fouth made it go in better.   Im going to wait unitl the drive shaft is installed before I add the fluid.  Its going to be a high temprature here in Madison WI of -10 on Monday.  So Im in no hurry to drive it...LOL    

Thanks
Steve69