What you actually want to do is to create a mask texture that follows the outline of the health bar shape you want and then apply it to the status bar texture of your health bar.
Lua Code:
local mask = Health:CreateMaskTexture()
mask:SetTexture("path\\to\\your\\mask", "CLAMPTOBLACKADDITIVE", "CLAMPTOBLACKADDITIVE")
mask:SetPoint("TOPLEFT", Health, "TOPLEFT", 0, 0)
mask:SetPoint("BOTTOMRIGHT", Health, "BOTTOMRIGHT", 0, 0)
health:GetStatusBarTexture():AddMaskTexture(mask)
For instance, that's my old odd shaped unitframe (rip "round" layout
):
And here's the mask I used for its health bar:
