this is a recursive implementation - loads as many revs as are needed - and could be trivially tweaked to return the referenced revisions inline..
 explain analyze WITH RECURSIVE mp_revs AS (
    select sequence as sequence, revision as revision from branchrevision where branch=492361 and sequence = (select max(sequence) from branchrevision where branch=492361)
  UNION
   select branchrevision.sequence, branchrevision.revision from mp_revs, branchrevision, revision where branchrevision.branch=492361 and branchrevision.revision=revision.id and branchrevision.sequence=mp_revs.sequence-1 and Revision.revision_date >= '2009-08-10 21:56:30.623339+00:00'
)
SELECT sequence, revision FROM mp_revs;
                                                                                                 QUERY PLAN                                                                                                  
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 CTE Scan on mp_revs  (cost=12367.71..12368.13 rows=21 width=8) (actual time=46.900..161.402 rows=4178 loops=1)
   CTE mp_revs
     ->  Recursive Union  (cost=1.40..12367.71 rows=21 width=8) (actual time=46.896..156.775 rows=4178 loops=1)
           ->  Index Scan using revisionnumber_branch_sequence_unique on branchrevision  (cost=1.40..122.42 rows=1 width=8) (actual time=46.887..46.889 rows=1 loops=1)
                 Index Cond: ((branch = 492361) AND (sequence = $2))
                 InitPlan 2 (returns $2)
                   ->  Result  (cost=1.39..1.40 rows=1 width=0) (actual time=46.854..46.854 rows=1 loops=1)
                         InitPlan 1 (returns $1)
                           ->  Limit  (cost=0.00..1.39 rows=1 width=4) (actual time=46.845..46.846 rows=1 loops=1)
                                 ->  Index Scan Backward using revisionnumber_branch_sequence_unique on branchrevision  (cost=0.00..29518.34 rows=21199 width=4) (actual time=46.843..46.843 rows=1 loops=1)
                                       Index Cond: (branch = 492361)
                                       Filter: (sequence IS NOT NULL)
           ->  Nested Loop  (cost=0.00..1224.49 rows=2 width=8) (actual time=0.021..0.024 rows=1 loops=4178)
                 ->  Nested Loop  (cost=0.00..1210.61 rows=2 width=8) (actual time=0.011..0.013 rows=1 loops=4178)
                       ->  WorkTable Scan on mp_revs  (cost=0.00..0.20 rows=10 width=4) (actual time=0.000..0.001 rows=1 loops=4178)
                       ->  Index Scan using revisionnumber_branch_sequence_unique on branchrevision  (cost=0.00..121.03 rows=1 width=8) (actual time=0.009..0.009 rows=1 loops=4178)
                             Index Cond: ((public.branchrevision.branch = 492361) AND (public.branchrevision.sequence = (mp_revs.sequence - 1)))
                 ->  Index Scan using changeset_pkey on revision  (cost=0.00..6.93 rows=1 width=4) (actual time=0.008..0.009 rows=1 loops=4178)
                       Index Cond: (revision.id = public.branchrevision.revision)
                       Filter: (revision.revision_date >= '2009-08-10 21:56:30.623339'::timestamp without time zone)
 Total runtime: 163.074 ms
(21 rows)