So you want to increase those ship's maximum hit points? No problem, look at the shield bestowal ability of the advent hangar. The tricky part might be getting it to behave right, as structures don't move and thus don't have the problem of its targets leaving the range. If you just want the extra shield points to immediately be dropped that's no problem. Also I don't think the extra shield points are granted immediately, rather they have to be regenerated as normal, which may compound the moving out of AoE problem.

If you want it to cover the gravity well, don't use range.

ApplyBuffToTargetsAtOrbitBody

Have the caster finish on entering hyperspace, and the applied buff finish on the same condition, along with when the casting buff is lost, and you have a solid setup.

I'm having trouble finding a buff type that increases shield hitpoints, i can find the one for shield restore just fine, but for actual shield hitpoints i can't see anything, could it be that that's not there? Or can i just not find it?

/vap

Edit:

I noticed the researchFloeatModifier called "ShieldPointsMaxAdjustment", can i use that in an ability?

Yes, you're right, i'm just being dense.. It goes:

overTimeAction
    buffOverTimeActionType "RestoreShields"
    shieldRestoreRate
        Level:0 4.000000
        Level:1 4.000000
        Level:2 4.000000
numEntityModifiers 1
entityModifier
    buffEntityModifierType "MaxShieldPoints"
    value
        Level:0 750.000000
        Level:1 750.000000
        Level:2 850.000000

in the advent shield buff, being up 20 hours and spending most of that texturing ships kinda takes away your focus, i only saw the top one, never even noticed the bottom one until i had a second look, silly me

/vap

Edit:

Next question, is there a way to make it a percentage value rather than an actual value?

Not with an ability, only research can do that. Someone else tried to do that recently, so I am certain of this. Best you can do is give a different amount of shields to frigates/capital ships/planet modules/star bases, but you can't do a specific percentage no.