Perhaps, but then it'd need a big oil reservoir to store the oil when the cuff is deflated.
I ran this question by an AI (I know, boo hiss AI, but it gives references for the stuff it said). It said there actually are some quiet and discreet pneumatic compression stockings that came on the market recently (the Venowave VW5 was mentioned specifically), but they go down on the calves rather than having anything mounted on the waist. It said it couldn't find any hydraulic equivalents available anywhere. It also says that Trump's diagnosed chronic venous insufficiency isn't something that would normally need pneumatic compression stockings like this, it would be overkill. These sorts of things are more often for post-operative situations when a person is bedridden.
Other suggestions the AI gave me were a very large phone in a back pocket (we of course know how addicted Trump is to social media on his phone) or a poorly-tailored bullet proof vest plate (for some reason Trump, despite being worth billions, dresses in ill-fitting off-the-shelf suits. Maybe a result of some kind of weird witch's curse that affects his bulletproof vest too). It also dug up news articles about other weird rectangular bulges under his clothing going back September of last year so this may not be anything new.
"And then it got worse" is the chorus for Russian history.