This has come up before and I've thus far resisted implementing this feature. The main reason is I am a bit of a security nut when it comes to marshalling. There's a reason why I didn't allow @Portable to be inherited. And I didn't add implicit marshallability -- I want it to be very clear exactly what model is exposed to the wire.
In general, with Errai, we try to err on the side of convenience, but this is one of those areas where we've treaded carefully. And the exemption of wildcards was quite deliberate.
That said, I'd consider adding this in conjunction with a new flag. But I'd also probably want to output a warning with all the implicit portable types on startup. =)
For our project, a new flag and an output would definitely be preferable. To deal with the increasing number of classes we need to list in ErraiApp.properties, we generate the list from a simple script, which (in hindsight) removes any incidental security that would have been found in manually adding the classes to the properties file. So maybe a list and a count of the classes made portable would be a improvement - I'd see tthe compiler output and notice a change in the total number of portable classes more often than I would look through the list of classes listed in ErraiApp.properties.