Ok, so from the Linux-side it looks like there's no way to restrict the drive letters assigned to samba shares. (I'm using Samba 3.2.5)
On the Windows side however I tested with the following:
net use H: \\test\home\pickup
net use /persistent:yes
After executing the above commands a new Registry Key was created:
I know it won't prevent users from mounting it in whatever path they want, but at least the samba share will always get automounted in a fixed drive letter.
BTW, sorry if this thread is starting to look a little Windows-ish. :P... I just put the Windows commands in the hope to help someone who is asking the same question around the internetz.
Thanks all for your help, I thought I could fix this from the Linux box but your replies put me in the right direction.