You can generate tracker items via an http POST to the tracker service, we have just started a doc page API tracker but it needs lots more work...
One problem you will face is authentication if this is to be a public tiki - you could use Token Access but that is quite hard to set up.
Maybe you could put the tiki in an iframe on the other app and just get them to register and login as usual? What is the other app? (and why can't that be a Tiki too! )
Sounds like a good challenge, good luck!