I agree it would be handy. In fact, I use AutoHotkey to let Alt-R signal W is down. Then when I press W, the key coming up stops it. It makes cross-country driving more enjoyable and you're not wearing out a finger holding down a key for 20 minutes.
;************************************************************
; ONLY FOR 7D2D
;************************************************************
#IfWinActive, 7 Days To Die
;************************************************************
; ALT-R is autorun. W to stop.
;************************************************************
!r::
Send {w up}{w down}
return
;************************************************************
; ALT-M - Hold Down Left Mouse Button
;************************************************************
!m::
Click, Down
return
Change - Added AutoHotkey config.