Interesting...what kind are you thinking? S/S doesn't seem like waistcoat season for obvious reasons. The traditional odd waistcoat is tattersall, but that's more F/W, and VERY english country gentleman. The other odd waistcoats I've seen are takes off of morning dress - like a dove or buff waistcoat worn with a navy suit. But maybe a cream linen waistcoat would be a nice addition to a brown linen suit....
The reason I said S/S is because the F/W line has been released and it didn't seem feasible to say "Hey, could you find and provide some waistcoats for December which meet the initial standards you've set even though you've probably not even looked at anything?" I agree that F/W are more the season and I'd love to see NMWA apply their aesthetic sensibilities to creating a set of waist coats that would fit within this context. I also think the idea of a button front sleeveless cardigan - which is what it looks like Greg may be wearing - could be cool. I reiterate that I would be willing to defer to his discretion. I do think a rather neutral five button, single breasted, lapelless, two pocket waistcoat would be cool though, especially if it's cut well.
Here in the southern part of the US I think the seasonality of waistcoats might be a bit up for contest though. Here in Dallas there are four to six months of the year where it is practically unbearable to wear a jacket of any sort. It can be done and certainly some still do, but I've seen plenty who do with sweat marks on the outside of their jacket. This isn't necessarily because of the wrong choice in jackets either - I've an unstructured linen/cotton jacket that I bought specifically with the hope of being able to wear it over the summer, and I've taken it off on days because I'd really rather not sweat through it. We average 74 days from June 1 to August 31 at 90+ degrees, 16 of them 100+ (again, on average). Fourteen of the fifty largest metropolitan areas in the country average over 50 90+ days during that 92 day time period. At the same time I feel under-dressed without a jacket. The advantage to a waistcoat then would be in that it allows greater ventilation to areas more prone to sweat. Whether or not this can be done in such a way as to minimize constriction and additional sweating elsewhere is a separate matter more indicative of whether the whole thing is a good idea or not.
There's also the observation that the odd waist coat without a jacket has entered our collective stylistic lexicon - though I'm not going to put forth much explanation as to why such a choice would be. I also won't argue as to whether or not such a thing has been successful so far, because I don't think many have been. Most of what I've seen either look like orphans - taken from a suit, or are some god awful pattern.
However, I wonder if it would be possible to design a waist coat that could work in this capacity and still be appropriate to pair with a suit or odd jacket. As you said, a cream Linen waist coat would probably be rather versatile (for an odd waist coat). Alternatively, I think other colors of linen might work with a cream linen suit and be capable of being paired with say a solid OCBD, dark jeans, and chukkas. There may be patterns in tropical wool jacketing that could do this. If I remember correctly there are some H&S fabrics that could be paired with cotton pants and loafers. I make no assertion as to whether or not this would actually work, but I don't have an a priori argument that such a thing must always turn out badly and I would imagine that NMWA are guys that could find a stylish counter example.