Hello Ian. I truly appreciate your help in trying to identifying this unusual piece. While your suggestion that this could be a re-hilted rencong blade sounds possible the tang on this does not arise from the back edge of the blade like a rencong, but is like those found on most blades from Luzon and is located only slightly off-center of the blade. The tang is also quite long as it does extend completely through the hit and the now missing butt plate before being peened over in the usual manner for a blade from this location. However, I can see where it might have been specially made for someone in the style of a rencong to suite their personal preferences.
A lot of the fun in these odd items is in trying to identify them after they have been found.
Best,
Robert