>>$4003, $4007, $400B, and $400F are kinda multi-purpose, though

I'm not sure about other assemblers but Dasm can define multiple names for single addresses :)
ie:

Channel_Priority        =   $4015    ; READ
Channel_Ctrl            =   $4015    ; WRITE





At the moment my PPU names look like this

PPUCTRL0                =   $2000    ; PPU Control Register #0
PPUCTRL1                =   $2001    ; PPU Control Register #1
PPUSTAT                 =   $2002    ; PPU Status Register 
OAMADDR 	        =   $2003    ; SPR-RAM Address Register
OAMIO	                =   $2004    ; SPR-RAM I/O Register
PPUSCROLL               =   $2005    ; VRAM Address Register #0
PPUADDR		        =   $2006    ; VRAM Address Register #1
PPUIO                   =   $2007    ; VRAM I/O Register

But I don't think PPUSCROLL sounds right ...