What kind of pressure are you trying to measure? Internal injection pump operating pressure? Or external fuel supply pressure TO the injection pump?
For the former, I am not sure where/how you would want to install a sending unit. It would need to be a fairly high pressure gauge. It would be interesting to watch the internal pressure temporarily but not sure how useful it would really be as a permanent installation.
For the latter: remember, there is no external supply pump (at least on a stock Volvo diesel vehicle) so pressure anywhere upstream of the injection pump is zero. Or actually it is negative, suction. If you installed your gauge at the IP inlet banjo it would read nothing all the time, unless you have add a non-factory electric lift pump somewhere?
Measuring supply fuel pressure like this is pretty common on many diesels that do have lift pumps and especially on ones with different injection systems that are in some cases extremely sensitive to insufficient fuel supply. Maybe this is what gave you the idea? Typically this would be on Dodges/Cummins with the VP44 or P7100 pump, Fords with the 6.0L Powerstroke, and GM with the 6.5L engine. On those kinds of vehicles a fuel pressure gauge is a great idea and very valuable since any loss of fuel supply positive pressure can mean instant catastrophic destruction of thousands of dollars worth of fuel injection system parts. Personally I have one on my Cummins rig and watch it all the time in a paranoid manner, AND I have a loud alarm in the cab that sounds off if supply fuel pressure falls below 5psi. This is after replacing a $2000 injection pump on the side of I-90 earlier this year.
BUT on the VE injection pump systems like the Volvos have, a fuel pressure gauge wouldn't really serve any purpose. The VE pump is highly reliable as a source of its own fuel supply and not sensitive to fuel supply pressure like the models above, thus doesn't rely on a lift pump at all. All it needs is an unrestricted fuel line and filter that it can pull through without excessive effort.