Do you have the option of green forming the tube network and sintering it into its final configuration?

MGKPO4 has eccelent adhesive properties and is very high temperature if the right fillers are employed. This is applied at ambient

A ceramic adhesive like Sauereisen 78 might do it.

Are you looking it as vacuum tight or just to hold it together.

If you like to hold it together you can use high temperature cement from Union carbide but they are not leak tight.

If you are looking for vacuum stability only guy I know have is Global Applied Materials and it bonds to alumina and aluminum nitrate and gives good vacuum stability up to 1000C I know of

Can You give more details, like length of that tube when assembled, and what is it that would flow trough at such temperature? It would not help You if this fluid can corrode sealant used, and any sealant would probably have different contraction and expansion rate coefficient than that ceramic, so it is bond to develop cracks in long use, so it would not be leakproof any more, and then You have to break whole system as repair would not be possible, right?

If I were You, i would make connecting joints with conical ends and threads, just like PUP JOINTS used in joining drilling tubes together or fittings that are used for joining copper tubes to valves or forks or radiators in case of central heating.

It can be surely sintered to great precision, and conical ends should provide leakproof joining. Since materials would be same, there would be no differences in dilatation/temperature expansion of joints so this should be stable enough to hold and there is no need for glues, so You can dismantle it also if necesary........

Of course, as somebody suggested, if it can be sintered in one piece, it would be the best, but still you need to have way to join it to something else on the ends, right?

Regards from Zagreb, the Capitol of Croatia, Europe!

http://www.aremco.com/

This company makes suitable products.