Might try using the dependson function in the cell that does the update....something like this
prop.mycustompropertie = "hi bud"
user.my_cell_to_update_db = runaddonwarg(dbu)+dependson(prop.mycustompropertie)
This way, every time the prop cell changes via human typing in something else, the update cell is triggered. I believe this will cause the runaddonwarg to fire...from there you know the drill.