If I recall correctly the reason we eat a Seuda on Hoshanah Rabah is it is the last day we have a Chiuv to eat in the Succah. Suppose it is raining and you can not eat in the Succah do you still have a Chiuv Seuda?
It's still Chol Hamoed, so you should probably have a nicer Seudah anyway.
The "Noheg K'Tzon Yosef" (pg. 298) cited in Nitey Gavriel (Hilchos Sukkos pg. 383) explains the reason for the minhag of eating a seudah on Hoshana Rabba to demonstrate our confidence in a victorious outcome of the judgment. Accordingly, the seuda should be held irrespective of whether it is in the Sukkah or not.