Note that the Sjaak II engine is configurable for drop variants. It could be that Nebiyu is too (as I know it to support mini-Shogi). CrazyWa is not configurable, but it is open source, (a C program), and can probably easily be adapted to play Seireigi: it is an engine designed for playing drop variants, and already supports Wa Shogi.
Since Fairy-Stockfish supports regular Shogi, I suppose it can be configured for other drop variants too. (Caveat: although Fairy-Stockfish is in general super-strong, it derives its strength in Shogi mainly from a trained neural network evaluating the positions. I heard that this increased its strength by about 1000 Elo over using the hand-crafted evaluation.)
Note that the Sjaak II engine is configurable for drop variants. It could be that Nebiyu is too (as I know it to support mini-Shogi). CrazyWa is not configurable, but it is open source, (a C program), and can probably easily be adapted to play Seireigi: it is an engine designed for playing drop variants, and already supports Wa Shogi.
Since Fairy-Stockfish supports regular Shogi, I suppose it can be configured for other drop variants too. (Caveat: although Fairy-Stockfish is in general super-strong, it derives its strength in Shogi mainly from a trained neural network evaluating the positions. I heard that this increased its strength by about 1000 Elo over using the hand-crafted evaluation.)