The other complication is that Inmarsat up-link is a steered spot-beam ( the antenna on the aircraft is stabilsed and driven, or phased-array in recent implemenations ). So the SIGINT sat would have to be passing through the up-link 'beam' precisely as it was transmitting.
Down-links are ( depending on generation and usage ) either from one of an array of narrow-beam antennae on the Inmarsat or from the wide-area general broadast antenna; much easier to intercept, you and I can do it with a sat-TV dish and an RTL dongle.
Brochures I saw years ago indicated that least some portable Inmarsat terminals are also directed beams. Taliban/AQ was known to use directional antennas for communications security, at least later in the war. So possibly we could say the same thing about Bin Laden's satphone, and we know how that turned out.
Down-links are ( depending on generation and usage ) either from one of an array of narrow-beam antennae on the Inmarsat or from the wide-area general broadast antenna; much easier to intercept, you and I can do it with a sat-TV dish and an RTL dongle.