Afaics, there can't be a layer outside the onion routing maintaining constant flow to outputs of the network unless you have masternodes in control of maintaining the constant flow, which would then be egregiously vulnerable to Sybil attack. How do you envision it working otherwise?
Thus constant flow is inimical to the obfuscation of fixed points through randomization, which makes a Chaum
mix-net work.
You can build what is basically mesh network with fixed capacity connections upon which you run the mix network. Whenever you open a connection to another peer in the network, you keep open the connection and both sides keep sending packets at the agreed upon rate while tunneling their actual communication through this constant stream. Packets are routed through this network to the target peers chosen for the onion routing route for the given packet.
I see that this design is probably more than can be easily implemented on top of I2P if it was desirable at all.
It occurs to me that this discussion is probably off topic here.