If one is on a tiyul on sukkos and it starts raining, I am presuming he is allowed to have a seudas keva. So when it stops raining but the sukkah he would’ve had would still be dripping, is he still allowed to eat a seudas keva for the time that it would take to stop dripping?
Yes, whether at home or on a trip, when it is raining into the sukka one is exempt from sukka at that time. This exemption continues as long as the sukka is not fit for eating, i.e. the food will be ruined from rain dripping in to it.
MIshna Sukka 28b, Shulchan Aruch 639:5. This is all due to the basic rule of “tayshvu kayn taduru” one is only required to eat in the sukka when it resembles hos home, i.e. it is livable. Just as one would leave a room in his home if it was raining in, the same applies to the sukka.