It wont fit into a blaster warlock's bonus feats or levelling needs. It wont fit with summoners who have warlock level 28 as a prereq for epic summon. I did put this together:

Which is a hideous blow warlock. It gains Bard Level 21 songs, which seems worth the sacrifice of three levels and a few feats. This would be a cool and effective build.
The issue I found was this:
Arcane Spell Failure (ASF) for Armor and Shields is reduced by 20% for pure-class Warlocks, or multi-class Warlocks with at 6+ levels in the class. This ASF reduction is automatically applied to equipped armors and shields.
Emphasis mine. What this means is you completely lose ASF reduction, even with eldritch armour. I don't know if this is intentional but this means you'd have to go to 6 bard, which means losing another warlock bonus feat. Which means you can't really manage a hideous blow lock, blast lock or summon lock unless you're willing to go naked.
Could the bardic synergy feat be amended to allow for ASF reduction to apply still on a bard dip?
EDIT:
I've done some testing and I think this is actually a bug.
27/3 warlock/anything else still grants ASF reduction
It may be that the bardic ASF reduction is interacting poorly with the warlock ASF reduction.