I've taken a stab at applying the idea of metamagic to martial adepts. I've found a few other takes, but they've been too complicated or allowed you to add powerful effects to maneuvers that are already of the highest level you can initiate.
Meta-ManeuverPrerequisite: one martial maneuver
Benefit: You learn two martial augmentations and gain one augmentation point per encounter.
Whenever you initiate a maneuver (but not a stance), you can spend an augmentation point to add a martial augmentation you know to it. If you have multiple augmentation points available, you can add more than one martial augmentation to the same maneuver (using one augmentation point per martial augmentation).
Each martial augmentation increases the effective level of the maneuver by an amount noted in its description. Any save DCs remain unchanged (unless using heighten maneuver). If multiple martial augmentations are added to the same maneuver, the modifiers stack. You can't add a martial augmentation to a maneuver that would put its effective level above half your initiator level (rounded up).
Special: You can take this feat multiple times. Each time, you learn two addition martial augmentations, and your number of augmentation points per encounter increases by +1.
Here are some martial augments based on metamagic feats.
Empower Maneuver
Maneuver Level: +1
Benefit: All variable, numeric effects of the maneuver (except saves and opposed checks) are increased by one-half. If the maneuver involves a weapon attack, the normal effects of the attack aren't affected, but those added by the maneuver are.
Energy Substitution
Maneuver Level: +0
Benefit: Any elemental (acid, cold, electricity, or fire) damage from the maneuver is changed to a different element (acid, cold, electricity, or fire) of your choosing. You can choose a different element each time you use this martial augmentation.
Extend Maneuver
Maneuver Level: +1
Benefit: All non-instantaneous effects of the maneuver last twice as long as normal.
Heighten Maneuver
Maneuver Level: Variable, see text
Benefit: Choose a number to add to the save DCs of the maneuver. This number is also added to the maneuver's effective level.
Maximize Maneuver
Maneuver Level: +2
Benefit: All variable, numeric effects of the maneuver (except saves and opposed checks) are maximized. If the maneuver involves a weapon attack, the normal effects of the attack aren't affected, but those added by the maneuver are.
Nonlethal Maneuver
Maneuver Level: +0
Benefit: All HP damage dealt by the maneuver is changed to nonlethal damage. If a maneuver involves a weapon attack, the normal damage from the attack is also changed to nonlethal.
Quicken Maneuver
Maneuver Level: +3
Benefit: If the maneuver normally requires a standard action to initiate, you can instead initiate it as a swift action. This has no affect on maneuvers with other initiation actions.
Rapid Maneuver
Maneuver Level: +1
Benefit: If the maneuver normally requires a full-round action to initiate, you can instead initiate it as a standard action. This has no affect on maneuvers with other initiation actions.
Searing Maneuver
Maneuver Level: +0
Benefit: Fire damage from the maneuver bypasses fire resistance. Half the fire damage applies to creatures normally immune to fire damage. Creatures vulnerable to fire take +100% of the fire damage (instead of the normal +50%).
Transdimensional Maneuver
Maneuver Level: +1
Benefit: The maneuver (and any weapon attacks made as part of it) functions across dimensional boundaries. This allows you to affect incorporeal creatures and creatures on coexistent planes as easily as corporeal creatures on the same plane. This doesn't give you the ability to see into coexistent planes.
Twin Maneuver
Maneuver Level: +4
Benefit: You initiate the maneuver twice in quick succession. You still only use one action to initiate it, and it is only expended once. Make all rolls and decisions separately for each instance of the maneuver. The two instances can be targeted at the same creature or two different ones. You can't take any action (not even a 5-foot step) between the instances.
