I don't know about "significant long flights", but the jet glider idea is certainly not new: I think the first turbojet powered glider was this one: the Fouga CM8-R13 http://www.minijets.org/index.php?id=67

Gérard Pierre was silver medalist with the unpowered version, Fouga CM8-15 at the world championships 1952 at Madrid Quatro Vientos, Spain.

Fouga made a whole series of derivatives, http://www.minijets.org/index.php?id=159 including this Fouga Gémaux, reminiscent of the Twin Mustang: http://www.minijets.org/index.php?id=74

And don't forget the pulse-jet gliders: the Emouchet Escopette in France: http://www.minijets.org/index.php?id=135 or the Bocian Puls in Poland:http://www.minijets.org/index.php?id=138.

In the US in the sixties to seventies (advert in Soaring july 1974, for example), Gluhareff produced pulse-jet units that could be used for gliders, and I recall an advertisement in Soaring in the same period with still another type of pulse-jet.