This is a tricky area, and I don't have time to do it justice but here goes ...
I believe this is a separate piece of fabric, worn, probably, underneath the doublet. In the accounts for Charles VII (1458-9) there is reference to cloth for. literally, a 'piece' ...
"une piece a mette devant son estommoc" I say underneath but this is difficult to judge, bt in similar, later illustrations it is clearly beneath the lacing of the doublet (e.g. a swordsman in on the St Ursula eliquary.
In later illustrations from France (e.g. Rene of Anjou's Book of the Tournament) and England (the Eton Murals) a 'stomacher' is being worn over the doublet ... this reaches to the throat, leaving only the doublet collar showing. Personally I think this probably a 'petticote' as described in Sir John Fastolfe's inventory ...
"Item j Pettecote of lynen clothe soffyd with flokys
Item j Petticote of lynen clothe withought slyves"
In John Russell's Book of Nurture the Chamberlain is advised to prepare, amongst other garments,
"...First hold out to him his tunic, then his doublet while he puts in his arms, and have his stomacher well aired to keep off harm."
In this case tunic is probably his shirt. This seems to imply the stomacher is over the doublet, but this may not be the case as the doublet is only laced later, so the stomacher is ready if required.
Also in the Howard's accounts for the 1466 there is
"Item, he same day my mastyr paid fore a stomakere of velvet, ij s. iiij d."
What is important is not whether such a garment was worn, but would it be worn in the context of the person you wish to portray. I'm not sure how widespread this fashion was ... or how specific to the wearers degree ... I'd want more research first.
You need to be wary of assuming that the colour of the doublet collar necessarily has to match that of the doublet itself. There are several references to a different (typically finer) cloth being used for the collar, and this is evident in several of the pastoral scenes of mid-fifteenth flemish Mss illustrations.
In the instances you mention the collar is a part of the doublet ... but (as the figure in the forground illustrates the gown could also have a collar .... then again it might not .... and then again it could be a half-collar.
As to whether there are other illustrations of this 'type' of doublet ... basically this is a normal doublet (probably with short puffed sleeves to padd the pleated shoulder of the gown worn over it. The fact that it is worn 'undone' is a matter of style in the wearing rather than in the construction per se. As it is a well-to-do gentleman the cut will be finer, so the loosening of the opening more pronounced but it would still be closeable (probably)
What I would need to know to be able to help more is precisely what date, where and what degree is the person you wish to make the doublet for ...
Hope this helps!?
Cheers
Dave