I believe this can be done by creating multiple patchsets for one issue, and 
using instead multiple issues to track different versions of the same patchset. 
 It is a bit different from the usual way you're supposed to use rietveld.  
This was suggested to me by a Googler, at least. :)

However, you'll probably encounter issue 267 and possibly issue 268 when trying 
to do this.

That sounds interesting and uploading multiple issues matches with my 
understanding of Mercurial's patch queues. Intersted in contributing a patch?

A patch to upload.py?

Yeah, the Mercurial backend in upload.py is the only place where we can deal 
with patch queues. It seems that the mq extension is widely used in the 
Mercurial world, so supporting patch queues is a reasonable feature IMO.

The same should be done with git then.  However, I am afraid that without some 
more specific user interface the number of issues created can be unmanageable.  
For many Mercurial and git projects it is quite common to submit patches a 
dozen at a time.

(In the process of learning MQs)

My opinion is that upload.py should simply use "hg qdiff" when available.

It is convenient to organize patches into trees regardless of MQ support. 
Consider scenario when overly huge patches stay in queue for too long just 
because nobody can deal with complexity. In this way a patch can be split into 
queue of patches that is possible to review separately.

When we can construct such "review trees" - we can tie MQ patches to them 
(extension or script - doesn't matter). As MQ patches can move relatively to 
each other, we will need a way to identify patch to find an issue for it when 
it moved or renamed.

What is the incantation sequence of commands to use upload.py when you've got 
your patch ready via "hg qdiff" today?

I'd love for upload.py to support at the very least that simple common case.

Can't you use --rev rev1:rev2 ? Or is the problem that you have no way to 
figure out what rev1 and rev2 are?

I dug the solution and posted to 
http://code.google.com/p/rietveld/wiki/FAQ?ts=1372672996&updated=FAQ#How_can_I_u
pload_a_patch_from_Mercurial_Queue but it is very-very hard to find.