[Dovecot] Conditionally use a sieve script with deliver?

[Kārlis Repsons]

I found a post in this list [0], which says sieve scripts can be used to 
deliver mail as read. However, can it be done somehow conditionally, I mean, 
with a direct or indirect switch for deliver?
For a nonexistent example, it would be simply 
< | deliver -m path/to/maildir -r > or maybe an environment variable could be 
exported to deliver sieve and checked there?

(Still not sure if sieve can replace maildrop in my case, however, using it 
with dovecot creates various problems with interoperability; but perhaps I can 
just add those imap4flags like "\\seen" myself to the message with maildrop and 
avoid mess with sieve at all?)

[0] http://www.dovecot.org/list/dovecot/2008-July/032294.html
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