[Dovecot] using local-part delimiters in LDA delivery?

[OpenMacNews]

hi all,

i've exim 4.53RC2 + dovecot-cvs on OSX 10.4.2.

i'm working on implementing recipient-delimiters for delivery from exim to a 
user's imap store using dovecot's cvs-lda.

reading on the wiki (Jakob's latest comments?):

" ... the part after the delimiter is passed to the lda as an additional 
parameter. so dovecot-lda should handle that parameter and leave the separator 
handling to the MTA. The MTA has to handle it anyway."

it looks likes it's possible ...

with appropriate changes in exim to my router:

	dovecot_localuser:
		driver                  = accept
		domains                 = +local_domains
		local_part_suffix       = +*
		local_part_suffix_optional
		transport               = dovecot_lda

and transport:

	dovecot_lda:
		driver                  = pipe
		command                 = /.../deliver \
								  -c /.../dovecot-deliver.conf \
								  -d $local_part@$domain
		message_prefix          = ""
		message_suffix          = ""
		delivery_date_add
		envelope_to_add
		return_path_add
		log_output
		user                    = MY_MAIL_USER

i'd like to be able to build delivery conditions in dovecot so that, e.g.

	mail to:	bob at mydomain.com
	is delivered to "bob"'s default INBOX.

but,

	mail to:	bob+spam at mydomain.com
	is delivered to any specified (either a server-wide common relative path, or a 
user-specified) SUBFOLDER, say  "bob/INBOX/.../.../spam_to_look_at" defined by 
a dovecot-rule invoked by the delimiter "spam" ...

this is a little different than the approach Mark mentioned of delivering 
directly to folder_name specified by the local-part-after-delimiter ...

questions:

(1) until the two conf files merge, i'm guessing there needs to be some 
conditional logic in dovecot-deliver.conf rather than in dovecot.conf.

correct? or should i be thinking SIEVE rules?

(2) exactly WHAT "additional part" do i grab in dovecot?

thx!

richard