The internal lift pump is supposed to move much more fuel than the engine uses; the excess is dumped out the return line port. If you try to plug that port, the port will rise to quite a high pressure.
The IP's internals can probably see a bit over 100 PSI in normal operation.
(reference: this post.)
Reading from a 1984 Toyota L/2L/2L-T FSM, part of the IP test procedure for a similar VE-style IP (upon rebuild) is to check the housing pressure at various RPMs.
Yup, that's 104 PSI peak @ 4400 engine RPM.
One consequence of plugging the return line is that the IP's internal advancing mechanism will fail to operate (assuming it's still working). The mechanism depends upon a differential of pressure across the timer piston. The return line is the "low" side of this differential.
Also, return line flow carries any debris away from the IP and back to the tank, to be caught by the filter system. Think wear particles.
So, merely plugging the line would not be a good thing to do.