I think that this is a great discussion point, Mirko. It is definitely something that I have had challenges with. I too have been working with WFA forever (in fact since the Alpha days of 0.6 [wow it has been forever]). Honestly, the introduction of reservations in 1.1.1 was huge. The limitation on the existing commands has made working with reservations very difficult. I have two major issues with lack of reservation support.
Writing new commands to fill in the gaps: This has become much easier as new releases of WFA contain more certified commands but I still have to write my own in other areas. In 2.1, we have many of the basic building block elements as certified commands. This makes development easier but what happens when I edit the command? As you pointed out, if I edit the command and save a copy then it loses its certification. The loss of certification means that I can't leverage reservations. I also agree completely that waiting for UM to refresh (UM6.0 doesn't even have a way to force a refresh [not as far as I have found] so this means a hard set 15min) does not work.
Custom content for non-storage components: Here is another area where we are lacking support. I do a lot of work outside of storage. I would say that at least 50% of the workflows that I create are tied to external datasources. Most of the time, I have to create my own custom datasources on top of that. This means that none of this content can laid in the reservations pool. As the product continues to develop, we are moving out of just creating basic storage building blocks and further into developing full service catalogs. The only way 'around' this limitation is to force a refresh of the entire datasource after every workflow execution. This is sub-optimal because of the time that it 'could' take to retrieve the data.
Mirko, thank you for starting this conversation. I agree that this is something that we should discuss and bring up with the WFA team.
I agree with everything that has been said so far. I'd love to have a way for adding reservations in custom commands and had a few situations in the past were this caused issues.
Not sure if "open source" is the correct term for this discussion as WFA itself is not an open-source product (and not intended to be) though it is very open. But everything that is used by certified content should be available to custom building blocks as well and reservations is the one big exeception we currently have.
Regarding "How hard can it be" - I'm sure there was a good reason to not open up this mechanism so there is likely some technical challenge to this. It should still be possible to do but there wasn't any non-open-source motivation included in the initial decision.