StringItem length

The "current" L4 X.2 reference manual (2006-11-17) states on page 56 
(section 5.4 StringItem) that the "maximum string length is 4 M bytes, even 
if the according field is 54 bits wide on 64-bit processors".
Two questions:

1. How do you encode a string size of 4 MB on 32-bit?
As far as I see it's not possible (only 22 length bits) and there is no 
implicit +1 used in the kernel source code. If this is intended (probably to 
allow zero-length strings (why?)), the manual should be more clear on the 
maximum size ((4 M - 1) bytes).

2. Who enforces the maximum size on 64-bit (x64)?
For intra-space string copies I didn't find any reason why e.g. a 10 GB copy 
shouldn't work. For an inter-space string copy still even any 1 GB copy 
should work, as the copy area has a size of 2 GB. I have no idea, what will 
happen to larger copies, though. I guess an unhandled kernel pagefault when 
the memcpy reaches the end of the copy area?

Best regards
Moritz