I did as you said - made a file called "Buildtags.cfg", put it in GameData/RP-O, with the following text

@PART{*]:HAS[@MODULE[ModuleAvionics]]:AFTER[RP-0] { MODULE { name = ModuleTagAvionics } }

Then restarted the game.  My RP-0_KCTPresets.cfg has the following - just the last bit is new.

KCT_Preset
{
    name = KCT Settings for RP-0
    shortName = RP0
    description = The recommended config for using KCT with RP-0
    author = RP-0 Devs
    allowDeletion = False
    career = True
    science = True
    sandbox = True
    KCT_Preset_General
    {
        Enabled = True
        BuildTimes = True
        ReconditioningTimes = True
        TechUnlockTimes = True
        KSCUpgradeTimes = True
        Simulations = True
        SimulationCosts = True
        RequireVisitsForSimulations = True
        TechUpgrades = True
        StartingPoints = 15,50,5000
		SharedUpgradePool = True
    }
    KCT_Preset_Time
    {
        OverallMultiplier = 25
        BuildEffect = 1
        InventoryEffect = 100
        ReconditioningEffect = 576
        MaxReconditioning = 13500000
        RolloutReconSplit = 0.5
    }
    KCT_Preset_Formula
    {
            UpgradeFundsFormula = 10000
            NodeFormula = (((5+((min([N],30)*min([N],30)*0.025))+(min([N],30)*0.025)+(max(0,([N]-30))*1.5)) / 86400 / 365) * sign(-[I]) * (1+([R] * 0.25))) // 5 science/year, + 0.05 to 1.5 sci per upgrade (levels at 30), only one rate, and 1/1.25/1.5x etc from R&D upgrades.
            EffectivePartFormula = min([C]/([I] + ([B]*(max([U]*0.125,1)+1))), [C])
            ProceduralPartFormula = (([C]-[A]) + ([A]*10/max([I],1))) / max([B]*(max([U]*0.125,1)+1),1)
            BPFormula = (10000 + ([E]*60))*[O]
            KSCUpgradeFormula = ([C]^(3/4))*200*[O]
            ReconditioningFormula = (min(7200+((([M]*[O]*[E]*[C]*0.001)^(1/2))*60), [X])*abs([RE]-[S]))*([L]+4)*([L]+4)/25
            BuildRateFormula = (1+([L]*0.25))*(([I]+1)*0.05*[N] + max(0.1-[I], 0))*sign([L]-[I])
            SimCostFormula = max([C]/50 * ([PM]/[KM]) * ([S]/10 + 1) * ([A] + 1) * ([L]^0.5), 100)
            KerbinSimCostFormula = max([C]/500 * ([L]^0.5), 5) // at ^.5, 15min = 30x
            InventorySaleFormula = 0
            ResearchFormula = -1
            UpgradeResetFormula = -1
            UpgradeScienceFormula = -1
			NewLaunchPadCostFormula = 25000*(0.75 + ([N]^1.25)*.25)

	KCT_Preset_Part_Variables
	{
		Module_Variables
		{
			ModuleTagAvionics = 10000000000
		}
	}

}