It could possibly work, but the better question is why would you want to test it this way? You'd be "grinding the gears" for no reason (even though it's the synchros you'd actually be grinding). This also would not provide a definitive "yes, my clutch is slipping" response. The reason for this is, even with a slipping clutch, in most cases the clutch would still have some grip to it, but not enough to provide full holding power. This means, up to the point where the friction disk breaks free from the pressure plate and flywheel, there is still some holding power being utilized. You would be forcing your synchros to try and cause the friction disk to break free, which would cause them an extraordinary amount of wear. The only way you'd be able to tell if the clutch was slipping would be if the synchros could hold everything until the clutch starts slipping, which really isn't going to happen. The amount of surface area the clutch has over the synchros is probably 1000:1 ... even with a slipping clutch, you won't overcome this disparity, so slippage most likely would never occur (in the vast majority of the cases*).
A much better way to test for clutch slippage is to:
- Have the car moving at a lower speed, along the lines of 20mph (32kph)
- Put the transmission in a higher gear than what you'd normally be driving in at this speed (probably 3rd or 4th)
- Give the accelerator pedal a hard go, giving the engine much more gas than you'd normally give it
- If the engine revs up without the car speeding up accordingly, the clutch is slipping
- If the engine speed stays linear with the vehicle speed, your clutch is not slipping
Remember, in any given gear, the engine speed to vehicle speed will stay relative and linear. As the engine speeds up, so should the vehicle. If the engine is revving higher than it normally would or if the engine gains speed faster than it should, the clutch is probably slipping.
Also, the reason you want to check for slippage this way with the car moving is an attempt to keep from lugging the engine, which is bad for the entire drivetrain.
/* As an aside: I had replaced a clutch in a Ford Escape where all the friction material was completely gone from the disk. You could use this test in this case with no fear of damaging the synchros, but I think you'd not need to do the test because there was absolutely no vehicle movement with the clutch engaged anyway./