Years ago, whilst working on a PARC contract, one of my slightly disenchanted colleagues pointed out to me, you do realise what PARC is spelt backwards. . . . .
well, I never found them to be, but this deal they are offering sure is.
It seems to me, with regards to Belgian IRS, that they do not look TOO hard into the affairs of expat airline employees, indeed some of my ex colleagues survived 5 or more years contracting there before awkward questions were asked of their employer vis a vis the temporary nature of a 5 year term. Read into that what you will, latitude or a brown paper bag? who knows. Lots of contractors over the years at JetAirFly ,Brussels Airlines etc, and not too many horror stories of knocks on the door in the middle of the night
AFAIK.
Offering this level of renumeration I guess they are figuring you pay your dues in a cheaper place and take advantage of a billateral tax agreement with Belgium, or are hoping no-one stays long enough to get caught, or most likely of all they dont give a stuff.
For sure you need to desperately want to fly this particular type a lot to accept these terms, much better avail in both seats flying a humble 737.
As has been said before, unfortunately they will find takers. I hope that they all have the common sense to use it as a mere stepping stone, and leave TNT with the maximum of contempt and the minimum of notice.