The problem with this approach is that you cannot interchange the oil used with R12, with the oil used with R134a.
This does not mean that your idea is without merit, however. If you are willing to flush the entire system and change the compressor oil after you have located your leak, then this MIGHT be a practical thing to do.
To begin with, R12 is coming down in price.
Your approach also depends upon what leak detection you have on hand or plan to use. If you plan on using UV dye and a black light, then using the 134 and flushing later may be reasonable.
If, on the other hand, you have a sniffer, you can rig up a Nitrogen bottle, put in 2 ounces of R22, then pressurize the system at about 200PSI with Nitrogen. This will put the entire system under pressure and your sniffer can detect the leak wherever it might be. This method is legal. The EPA figures that it is better to have 2ounces of R22 in the atmosphere than to have a complete charge of R12 or even R134a.
I totally understand your wanting to take whatever measures necessary to conserve your remaining R12. I am down to about six pounds myself and I hold my breath every spring hoping that my 124 evaporator hasn't sprung a leak.