Yeah atomic swaps should do the trick for payment state (a liquidity provider can verify your plasma state and swap it for state they have on a different plasma chain). This is easy to reason about for monetary transactions (I’ll swap plasma chain A tokens for equivalent plasma chain B tokens), but harder to reason about for smart contracts (system probably has to mass exit and reboot on a new chain which isn’t ideal). This assumes that chain A and B are sophisticated enough to implement HTLCs.
Atomic Swaps - There is already a proposal to make fast withdrawals in a similar way, this should be extensible between two plasma chains as well.
This could alternatively be done with perun style “virtual channels” where you could swap chain A tokens for chain B tokens with state-channels. If both chains can implement a multisig account that exits to a state-channel contract on the parent, then two parties could agree to swap plasma tokens. This should get you to the point where you can deposit on chain A and then hop to chain B → C → and then exit on chain D. (in theory 
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