I think long form fiction can be workshopped successfully, but it depends on the nature of the workshop and the structure of the class. In an opt-res program, where people have jobs, this may not work so well. But in a normal residency program, why not? I used to have to read a novel a week for some of my lit classes, and when I sit down to read, 300 pages is the work of an afternoon at a coffee shop, really.
In a full workshop setting, it may be a bit much to go novel by novel, week after week. But if you broke a 20 person workshop into five groups of four, you could workshop five pieces simultaneously, and what's more, you could do task oriented editing sessions. Say one month is spent on overall structure, or plot. The next month on characterization. The next on dialogue. The next... etc...
By the time the year is over, each long form work has had five or six close reads, with all the editing and polishing that comes along with that.
You can keep the groups the same, or change them up. There are pluses and minuses to both. By keeping groups together, they become intimately familiar with each others work, and can better comment on each. By switching up the groups, you keep things fresh, and prevent small-group-think from setting in.
All that is required is that students have a long form work already prepared before the workshop begins, which can be ascertained at the time of registration. Call it a pre-requisite, if you will.