Ah.... yeah there's a thought pm, didn't think of that....
(goes off to try something)
 
@DsRaider - try this mate:
<GameItemType InternalName="Test_Airwalker">
  <DisplayName>Test_Airwalker</DisplayName>
  <Description>Horses grant your units more speed in combat and in travel and make them immune to being knocked prone.</Description>
  <Type>Mount</Type>
  <CanBeEquipped>1</CanBeEquipped>
  <AdditionalTrainingTurns>20</AdditionalTrainingTurns>
  <ShopValue>120</ShopValue>
  <GameModifier>
    <ModType>Unit</ModType>
    <Attribute>AdjustUnitStat</Attribute>
    <StrVal>UnitStat_Defense_Pierce</StrVal>
    <Value>5</Value>
    <Provides>+5 Defense, harder to hit</Provides>
  </GameModifier>
  <CutSceneDataPack>HorseMountCutScenePack</CutSceneDataPack>
  <IsAvailableForSovereignCustomization>0</IsAvailableForSovereignCustomization>
  <AIData AIPersonality="AI_General">
    <AIPrefType>AIPrefType_MOUNT</AIPrefType>
  </AIData>
  <ArtDef>Test_Airwalker_ArtDef</ArtDef>
  <GameItemTypeArtDef InternalName="Test_Airwalker_ArtDef">
  <GameItemTypeModelPack InternalName="Test_Airwalker_Default">
    <IconFile>HorseEquipIcon.png</IconFile>
    <TintR>0</TintR>
    <TintG>0</TintG>
    <TintB>0</TintB>
    <EquipSFX>Equip_WoodenItem_01</EquipSFX>
    <MountedAnimationPack>SoldierAnimationPack</MountedAnimationPack>
    <GameItemTypeModel>
      <!--<ModelFile>gfx/hkb/Monsters/M_Horse_Mesh_02.hkb</ModelFile>-->
      <Scale>1</Scale>
      <SkeletonFile>Gfx\HKB\Monsters\M_Horse_Skeleton_01.hkb</SkeletonFile>
      <Texture_Skin>M_Horse_Texture_02.png</Texture_Skin>
      <AnimationPack>MountedHorseAnimationPack</AnimationPack>
      <Attachment>Unit_Mount_Lcf</Attachment>
      <Color_Skin>70,50,35,255</Color_Skin>
      <ShadowImage>/gfx/decals/Shadow_Horse_01.png</ShadowImage>
    </GameItemTypeModel>
  </GameItemTypeModelPack>
  </GameItemTypeArtDef>
 </GameItemType>
 
Give it to a unit.... should lift them horse height of the ground.  Change the item <Scale> to make them higher.
Fairly sure that's exactly what you want.
Edit -> With that example, the guys feet will slide around slightly now and then.  To improve it I think you'd want to replace the "MountedHorseAnimationPack" above with a custom one and remove the horse animation that causes the problem.  Should be fixable though.
This could be cool if it works.
Another Edit -> Just did some more testing, could be a few things to iron out but I think it works.  *Might* need to add a modelfile back in.. I think there might be some weird things happening without one... needs more testing.