Just received your friends invitation, but apparently I'm only able to respond after being a member over one month :-(
I'll have to wait some days until I can respond thru PM.
Meantime I will prepare to try and write a clarifying message about the difference between DeSaegher (US way of writing names with capitals), Desaegher (French, Dutch and Flemish writing without a second captial "S") and "de Saegher" (two words without a capital D, also French, Dutch/Flemish).
I'll send it thru PM as soon as the forum rules allow me to.
Meanwhile I will try and collect some websites about possible ways where to search in Belgium and France for the name Desaegher.
I would tip on Belgium, West-Vlaanderen region. The name De Saegher can be found in that period pretty often in the area of Poperinge. I couldn't find him in that city but would recommend checking the decennial tables of surrounding villages. I also checked Beveren-Ijzer and Roesbrugge-Haringe already, because I found a few Desaegher's there, but no Joannes Baptiste.