WS and GW running on the host as VMs.
An I2P router is running on an other machine, it can be reached from the host.
Is there a way (preferably safe one) to let the WS use the I2P router for eepsites?
In theory, yes.
There once was a similar idea, i2pBOX:
Whonix-Gateway in theory could accept these connections, forward them elsewhere using an encrypted tunnel (VPN/SSH or so), where they get answered by i2p.
Sorry, I can’t provide step by step instructions. Would require too much research and testing. Too much workload.