Traffic rights are indeed not transferable, but the point is that when you split a company which owns traffic rights, there are no rules as to which part should have what rights; there is no rule that says each part must have an equal share of the traffic richts. in short, you can split your company (SN/DAT) in 2 parts, one (DAT) having all the good things like the lots, traffic rights, etc. etc. and one(SN)having only the depts.
It is not easy, but it is not impossible eighter.
Besides should you encounter some legal problems,
who gives the traffic rights: the government,
and who owns Sabena: the government,
and who owns DAT: the government,
you see how easy things can be?
Remember, for flights outside the EU we are still talking about bilateral contracts between 2 states each appointing their designated carrier(s).