Inexplicable "Inflatable Planets" - Hot Saturns

Back in September 2006 New Scientist ran an article on "The riddle of the inflatable planets" in which Dimitar Sasselov said that the newly discovered planet HAT-P-1 was so light that it would float on water "like a beach ball."

Now New Scientist has reported today that another group The Trans-atlantic Exoplanet Survey (TrES) has found another particularly anomalous planet designated TrES-4, this one being "about the density of balsawood" [Balsawood! Does it have any uses other than model aircraft construction?]

Apparently, according to New Scientist, Travis Barman, a theoretician at Lowell Observatory, said, "This planet cannot exist.", i.e. there's no good explanation for how it could have become so large.

However, here's a hypothesis: the planet is not as large as it appears - the apparent size of the planet suggested by the observed dimming during transit could be due to a set of sufficiently dense rings. Saturn's rings are considerably wider than the planet, and they scatter quite a lot of light (though I'm not sure how much... I may be a geek but I'm not actually doing this research!)

I see two basic possibilities for the existence of rings around such "Hot Jupiters" (planets of Jupiter mass very close to their parent stars)

1. There could be a stable ring system. Intuitively this seems unlikely given, amongst other things, the considerable radiation pressure likely to be experienced so close to a star - dust would be driven off

2. There is a temporary ring system - it would wouldn't need to be stable, it could be recently formed. Perhaps if the planet had one or more moons, additional tidal heating from stellar proximity etc. might have expedited its disintegration and the formation of rings, in which case extended observation might record progressive dissipation of the rings and decrease in the dimming observed.

In either case, what would have been observed would be a "Hot Saturn" rather than a "Hot Jupiter", and the abnormally low density would be explained by the mis-estimation of the planet's size, i.e. mistaking the attenuation due to the ring system for attenuation due to the planet itself.

I wonder how this hypothesis could be observationally tested - actually I wonder whether it is viable at all... but that's what a bare hypothesis is: an idea for exploration...