Can't really think of a way to do this with a custom workout. Don't think it's possible to set a minimum.

Maybe someone else can answer.

Its not a solution for setting a workload on your trainer but for visual reminder you could just make a long workout with one interval at your minimum wattage you will get a message to bring your power up.

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Bummer...no that wont work. I also want it for example when you are descending-to maintain 220 watts during a decent is very hard, legs need to spin very fast!

No.
But what sort of minimum are you talking about?? I don't find when on a free ride that mine will really drift low, even when on 3-3.5 hour sessions, unless I am consciously having a rest or are on a descent, in which case without erg mode it is hard to generate huge watts.

Pretty much exactly this. For example I might want to do a 2 hr ride with an average 220 watts that includes Alpe Du Zwift. The decent of Alpe takes forever and can put a massive dent in your average, and as you alluded to very hard to generate big watts.

Would be awesome if there was a mini feature so that no matter what happens you will achieve that amount.

It is not exactly what you are looking for but to overcome the lack of resistance during long descents you could always use the Zwift Companion app and switch between Erg and Slope mode. For rides were I want to make sure I am putting out consistent watts and not fluctuate much I will flip over to slop for the ride. I find with slope mode I am less likely to let my watts drift down.

You can probably combine an Ant+ Receiver with a battery and an Arduino then program it to shock you at various thresholds...

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So why not create a 2hr custom workout with your desired wattage on erg mode. You will hit a higher wattage climbing. Once you reach the top, toggle your wattage a little lower with your keyboard. Stay in the right gear and you should be good? You'll be prompted every time you need more power if you drop below.

Riding up should not be a problem to maintain minimum watts as each segment displays your average. You could just end the ride when you summit and not have to worry about going down. Plus if you link to Strava it will show your average watts for the segment.

If you really want to ride back down, set the difficulty slider to zero when you summit or as others have suggested use slope mode on the way down. Personally I like coming back down at the after the summit, pretty much free XP and drops if you just pedal a little.

Because I want the freedom to be able to attack a hill or a segment