As many have said, building a general purpose trigger box would be hard and you should just get a better scope.
BUT - I have often found it helpful to build custom logic into a circuit to aid in debugging and this could certainly include detecting events that one would like to trigger a scope from.
That's what I had to do for this thing at work, an experiment to see if the relay contacts in a piece of equipment would arc under worst-case conditions. I had two 277V AC sources, one a programmable supply and the other a variac running off wall current. The two supplies were out of phase with each other: the output of the variac "drifted" along with the wall current frequency (so there wasn't even a reliable phase relationship between the two). And the programmable supply, while its output frequency could be changed in 0.01Hz increments, had no means of simply synchronizing to the mains input frequency. I needed to switch between the two supplies (using a relay) when the two AC sources were 180° out of phase, and at the peak of their waveforms (so there was approximately 800V potential between them: I was trying to cause a failure).
So what I really wanted was this trigger:
TRIGGER = (PHASE(CH1, CH2)==180) & (CH1 >= 390V) & (CH2 <= -390V))
Which, it seemed to me, I should be able to generate using my oscilloscope's measure/math functions.
In the end, I built a little detector board out of two transformers, some diodes, voltage dividers and a quad op amp, and a little Microchip 8-bit MCU with ADC to generate the trigger. It worked perfectly, but... yeah, I guess I wished I had a really low-latency "general purpose" piece of equipment that I could just program to do this.
Oh, and even with all this effort, I
still wasn't able to make the relay fail...