I forgot to addess one more of your comments:
 
> (BTW -- this discussion about DSCP reminded me that the spec doesn't
> say anything about the ECN bits...)
 
That was in fact intentional. The LWAPP and CAPWAP specs never got into 
specifying how the ECN bits are to be used. I would recommend that we leave 
the use of ECN for a subsequent version of the protocol. At this point we can 
either continue to be silent about it, or simply add a statement at the end of 
section 2.6.1.2 to the effect of:
 
<possible text>
The CAPWAP protocol does not make use of ECN [RFC3168], therefore the ECN bits 
in the DSCP field MUST be set to zero (Not-ECT).
</possible text>