Bloody hell have we got an hour or 2?
I'll do my best...
FOr a start, no you can't tune a common rail diesel manually because there is nothing physically to change on the engine maybe you could fool the map sensor but that is would still take some kind of electrical intervention before you could turn the fuel up ,some first generation common rail engines like the Peugeot/Citoren DW10 HDI 8v used a wastegated turbo that wasn't controlled by the ECU (at least the 90 hp variant didn't but I'm not sure about the 110 hp) so you could tweak the actuator or even install a boost control of some sort, theres loads to choose from and its whole other subject to cover. Even some early VNT/VGT equipped engines could have the vane actuator altered to create boost even quicker but at the expense of making the power delievery harsher, first generation VNT's used vaccum actuators and a soleniod for boost control (ECU based) new generation ones have an electronic actuator thus removing the vaccum middle man.
Mechcanical fueling does have it's merits and draw backs but there is a bigger case for using over electronics for making power than you might think....
Davalav our cars uses what's called a rotary style injection pump,
This means it has only one plunger assembly to supply all the cylinders (unlike an inline injection pump which has one for each cylinder, I'll cover this some other time) to control fuel quantity there is a mechcanical governor which moves a collor up and down the plunger to control fuel flow (not pressure) fuel pressure is governed by a cam plate which will also control injection duration, each cam plate is cut for a specific engine and ours being an IDI engine requires relatively low injection pressure (175 bar for Bosch pump and 150 bar for Lucas pump) and long duration and the opposite for a DI engine.
Increasing the maximum fueling just adjusts the stop for the fuel collor to travel.
That was a breif description of what happens in the Bosch VE pumps
as for making power, well the Bosch VE can be modified in various ways, you can shim the governor spring which then stops the governor from pulling fuel at the top end of the rev range, you can you a larger plunger which allows you to move more fuel per stroke, larger delivery valves there are other things also like injector nozzles can easily be enlarged as cars using the VE type pump will use similar injector designs and the nozzles can be interchanged.
Also to note the Bosch VP pumps work in much the same way just with some electronic control...
This doesn't even scratch the surface on the subject really and it's very hard to explain on here,
If theres something I haven't mentioned that you wanted to know about just ask away and I'll do my best to answer