I currently got similar requirements for a new scope and just wait for a good price for an 3406D MSO.
Not triggering on a bus event pretty much kills the Pico for me right now.
Well it triggers on edges, pulses and so on.
The decoding is not in the device, but only very few MSOs or even logic analyzers can do that, most only as an option. It is nice to have, because it means you need less sample memory, the more specific your trigger is, on the other hand you got so much sample memory with the picoscopes that it´s unlikely to lose the data as long as you can narrow in a timeslot or another event just enough and search for it in the list of events afterwards.
A workaround could be to build a detector circuit for the very case which needs to be triggered and provides an interrupt for it, once i am messing around with protocols on physical level i might have the hardware for it laying around anyway.
It could be as simple as a (long) serial to parallel shift register with an AND mask once you get the clock right, which is the hard part in all bit banging. For the picoscopes there is also an API with less limitations than their software to fetch the sample memory, so it is possible to get quite some data. As long as we are talking about the typical protocols that run within 100MHz Bandwidth. (MSO channels: 100 MHz Bandwidth, analog channels: 200 MHz for an 3406D MSO; also: no BNC trigger input for MSO models, only as digital channel)
The problem with triggering serial data is that the trigger you need usually does not exist and needs to be coded anyway, as they can be very specific. Admittedly, implementing a protocol stack just for triggering is a tough nut, so i´d rather go with full controllers for that protocol in hardware as a development tool.