He already said he would be happy with the accuracy of the Siglent. So I fail to see why any fixed resistance can not be used to transfer a reading from that to the HP he is repairing. Of course you would not be at the accuracy of the HP. But you could work in a climate controlled room and go back and forth between the two. Read the value on the Siglent and then on the HP. Then on the Siglent again and on the HP again. And back to the Siglent. If all five of those values match, +/- one least count
of the precision level of the Siglent, then it is safe to assume the two match,
within the stated accuracy of the Siglent, of course.
Care should be taken to not change the temperature of the resistor being used. And good quality components should be used to connect them to the two meters. But it should be do-able.
Doing this with several similar value resistors would add to the confidence level.
For just comparing 2 meters one does not need super accurate or stable resistors. It still helps if the are reasonable low TC (e.g. < 50 ppm/K) so that temperature changes are not that critical and also self heating from the test current is not changing the resistance. Some meters measure some resistors (e.g. 1 K) with quite some power. With not so good resistors one can get reduce the error with a few repeats / cycles, but this can be tricky for the cal procedure to do the adjustment.
For the voltage transfer one can often measure both meters in parallel and thus get away with a simple source, like batteries. It may still be tricky for 300 V.
For the HP3468 to SDM3055 comparison part of the problem could be the different ranges (2 V vs 3 V).