Warlocks do have a few SLAs, and those are affected by metamagics (for free!). As are their normal spells (not for free).
edit: the following is a quick list of SLAs from the warlock trees. Some are CC (quicken, heighten), some are DPS (empower, maximize, intensity, etc).
Tainted Scholar tree:
Command SLA
Stunning Blast
Confusion SLA
Mass Confusion SLA
Soul Eater tree:
Devour the Soul SLA
Consume
Stricken
Consume Sight
Dark Feeding
Burning Blood SLA
Steal Life Force
Eldritch Wave
Finger of Death SLA
Enlightened Spirit tree:
Resist Energies
Eldritch Burst
Spirit Blast
Displacement
Acolyte of the Skin tree:
Fiendish Glare
Glare of the Pit
Some of their normal spells and pact spells are worth meta-magic-ing too (mostly just quicken and heighten), like Web, Soundburst, Tentacles, Mass Hold, etc.