Thank you all for your comments.
I may give the PCB repair a try; I'll have to see if I have a steady enough hand to fix traces without accidentally soldering everything together. I am certainly worried that the traces are too thin, too- even though they are very thin within the ECU, I expect that power transmission or signal quality will degrade if I make the leads minuscule.
When I bought the CCB, I did not even realize that there were different quality or thicknesses available. I just bought something with a low cost and a large enough cross section to satisfy my design.
I chose HCl as the etchant over Ferric chloride since it is supposed to be recyclable (and proper disposal of ferric chloride is troublesome). Also, I like the emerald green color of the CuCl2 solution.
My plan for a single sided pattern is the result of trying to keep the design simple. In the ECU, there are multi-layered connections (allowing for greater area to get between the PCB and the ECU header). I don't think that I have the capability to create vias and buried traces, so I'll just solder on the opposite side of my headers and terminals with a single-plane pattern. I have access to a jewelers drill press, so I was not anticipating too much trouble with creating the through-holes: just a bit of tedium.
In the mean time, I've submitted a request for quote to 4pcb. I'm trying to keep this cheap, but I've used a few months of hobby time just trying to learn to transfer a toner mask (as well as buying supplies for this, on the order of $50-$90 of transfer paper and transparency paper, PCB mask, copper clad board, etch chemicals, etc.). It may have been cheaper to simply order from a PCB production house from the beginning.
My intent with this project is not to make ECU access easier, rather, I am interested in treating the ECU as a microcontroller that I can play with and augment, much like an Arduino. For instance, the T4 has two unused analog input pins, so I am planning on directly connecting a TMAP sensor as well as a linear oxygen sensor (as opposed to the OEM logarithmic response sensor). I have not checked, but I also expect that there are digital IO pins available. There is a lot of unused flash memory available for patching in code, so I will be able to play around a bit with engine monitoring and controls. The OEM wiring harness is very expensive, so I don't want to permanently modify it.