No. the VDC RAM uses BITs not BYTEs, so if a particular registers uses only the first 4 bits, all other 4 "bits" doesn't exist on hardware.
The unused addresses are empty. In late 70's RAM was a precious resource.
What you can do, if you're not using GRID graphics, is disable them and use grid registers to load and store values. That's extra 16 bytes only.
Beware, not all registers are R/W, most of them are write only. I not sure exactly which ones you can read back.
There is a way to add extra RAM on a cart, but from what I know, it was never done before.
The internal RAM are 64 bytes, but the first 32 are used for processor :
16 bytes for registers (0-7 normal and 0-7 alternative)
16 bytes for Stack Pointer, you can use some of them if you know exactly what you're doing (subroutine levels deep and such)
Note the last 3 bytes is used for BIOS :
03Dh : Hold collision results
03Eh : Timer (count up to 60 then resets, 60 frames = 1 second, thus resets every second)
03Fh : Hold video status (not much usefull).
So you actually have 29 bytes free in the internal RAM.
I think for 8048 you're unique option is assembly.