Our oven has a matzav Shabbat, its programming is separate from the non-matzav Shabbat programming (their switches are unrelated, once Shabbos mode is on, the knobs belonging to the normal oven have no more effect — to the best of my knowledge). However, Shabbos mode does not use a different compartment of the oven, the very same compartment used for cooking, is used as Shabbos mode. Could one use this oven on Shabbos, as a plata, for placing dry food that is mevushal kol tzorcho?
If the degree of heat of the Shabbos mode oven is not sufficient for cooking (in a reasonable manner), but only for heating and for preserving heat, it can be treated as a regular electric plata (hotplate).
The fact that the compartment is used in a different programming mode during the week will not cause this to be prohibited, because the prohibition of mechzei kimvashel (it looks like you are cooking) can be interpreted as a concern that the person himself will come to cook, and this concern does not apply where it is not normal to cook.
Even if the problem of mechzei kimvashel is interpreted as a question of maris ayin (see Shita Le-Ran, Shabbos 40b; Penei Yehoshua), it is well known that some ovens have a Shabbos switch, and therefore there is no concern of maris ayin.
Some authorities permit placing food directly on a hotplate that cannot be generally used for cooking, whereas others require an overturned pot be placed on the hotplate.