IFR in the US is flown in controlled airspace. Otherwise, we generally get the altimeter setting from ATIS or ASOS radio frequencies, one of which is available almost anywhere. The differrence is that ATIS is recorded hourly by a human, ASOS is entirely automated and continuous. Neither involves talking to a controller although controllers can and will provide the altimeter setting too.
I fly IFR a lot of the time OCAS (in UK) and I use the ATISes from the nearest airfield in exactly the same way, or obtain the local QNH setting from a controller. As well as being more accurate, observing the changes in en-route QNH sometimes helps give an indication of the passage through a weather front.
Regional Pressure Settings? Using RPS under controlled airspace has resulted in a number of airspace busts, many of them by aircraft climbing out from low level. It's a system designed aeons ago, to help keep pilots above terrain when out of ATC contact for long periods.
Bearing in mind that by nature the RPS is a forecast for a large area and may be quite inaccurate (and pessimistic), I now use it only if I can't get a relevant QNH to use instead.
UK ATISes are updated more often than hourly, depending on the changing weather.