What device limits output current such that input voltage is maintained above some limit?
Rereading your question I think you are looking for something like this that is downstream from your logic circuits and before the power circuits.







simulate this circuit &ndash; Schematic created using CircuitLab

The top part with R3 is your classical current limiter.

R2, D1 and Q3 will begin to turn off the tail of the current limiter if the input voltage falls too low. The values will be rather soft though since you have no solid reference with a sinking rail.

As with all current limiters of this type there is a significant voltage drop incurred across the R1, Q1 circuit. 

Q1 and R1 would of course need to be powerful enough to carry your full load current and worse carry large currents with a higher Vce when it is choking the current. 

I do not think I would use this circuit for anything but lower level currents. Further, if you intend on using more than one of these on the same power bus, do not be surprised when they start an oscillation between them.

Ultimately you would be better to fix your load problems and or get bigger supplies.