I've been following the papers a bit, but I'm also not an expert in the field, so I likewise have a hard time assessing the plausibility of outgassing given the lack of an observed coma. I can't find a reference, but I seem to recall reading that there are still possible (though implausible) scenarios (depending on grain size & albedo & lots of other variables) where outgassing could be significant enough to produce the observed "thrust" but not enough to be visible in our scopes. Of course it could also be a combination of outgassing and solar radiation.
I will say though that physics people I've spoken to about it are much more skeptical of the solar radiation hypothesis, simply because radiation pressure is so weak. I'd be surprised if it were a thin slate of rock as the dimensions are so outlandish compared to anything we've ever observed, plus I would think be there might be "structural integrity" problems with an asteroidal body that thin and sheetlike. The "solar sail" calculations done by the folks linked to above may also assume/require that the sail "turns" itself somehow to constantly face the sun, since the observed acceleration vector was entirely pointed radially away from the sun.
I have heard one radiation pressure idea that I liked, though, which is the "dust bunny hypothesis". If 'Oumuamua were, instead of a solid body, more like a very loose, fluffy agglomeration of dust, it might just have enough surface area for us to see it (and to be hit by enough sunlight to get a push), while also having a low enough mass to produce the observed acceleration. IIRC this may have other problems, though, eg. requiring the dust to be super-high albedo for it to be visible. Seems like there aren't any "nice" answers left, so sifting through the "least improbables" is pretty difficult :)