I highly doubt you'll see anything across the north leg of the Beltway (between 270 and 95). Too much Rock Creek Park and right-of-way impact. There's also the Navy and DoD saying a hard NO to using any of the Naval Hospital property, which would push even more impacts into Rock Creek Park. These are also facts to face.
I think you are correct, though I find it amusing that some of the same Montgomery County politicians that were involved in frantic opposition to MD-200 (ICC) less than 15 years ago are now saying that is the "alternative" to widening (never mind that they and their allies at the Sierra Club, M-ICC, 1,000 Friends of Maryland and Virginia's Piedmont Environmental Council used to repeat, over and over and over again, that MD-200 had no benefit for Capital Beltway drivers and would never have any benefit for Beltway drivers).
Regarding lands along the I-495 right-of-way, there's not much between the Mormon Temple and the big bridge over the Northwest Branch gorge, though there might be enough to do some minor work.
It's not Rock Creek Park itself that is the issue
per-
se but the funding source that paid to purchase the land - the Capper-Cramton Act first enacted in 1930 and amended and expanded in 1946. Much of the Capper-Cramton Act lands are not owned by the National Park Service (though some are), but by other public park agencies (all of Rock Creek Park in Maryland is owned by the Maryland-National Capital Park and Planning Commission). Still, the federal government has residual powers over these lands. When I-495 was widened in the late 1980's and early 1990's to 4 lanes each way, MDOT/SHA was allowed to purchase some of the land from Rock Creek Park to make room between MD-355 and the temple. But SHA also had to agree that they would not be able to widen beyond that, which means the only way to add capacity here is with a viaduct (probably in the median) similar to the one on I-110 (Harbor Freeway) in Los Angeles County, California.
And of course, Section 4(f) of the USDOT Act of 1966 still applies.
My own personal opinion about this is more radical. Keep the 8 lanes (with some upgrades where possible), convert the entire top side of I-495 (maybe MD-187 to MD-650) to a priced roadway, with free passage to transit vehicles and HOV-3 traffic. That would really put the politicians that were opposed to the added lanes on the spot.