Yes, you can certainly make a pourable custard in the oven. The difference between pourable and firm custard is only in the amount of eggs. The custard will get done nevertheless, at the same final temperature reached. You will have to wait for a few hours instead of having it done in a few minutes, but it will work.
The time and temperature are exactly as for the firm custard. The temperature should be around 100 C, maybe up to 120, and the time is until you hit 83 to 86 internal.
What you can't do is a pipeable custard. For that, you need enough yolks (and other fat in the recipe) to make a very thick custard, but you cannot let it set firm. So, you have to stir it constantly.
Seeing that there are similar questions cropping up, a few more general words: Your food does not know if it is in an oven or on a stovetop, or somewhere else. What matters for it is 1) the pattern of heat transfer over time, and 2) the right agitation. The two do interact to some extent (in the example of this question, you can heat a custard to a certain final temperature by either adding hot milk to the eggs and stirring really quickly, or by sticking the mixture into the oven and waiting until everything has been heated up), but typically, it is the agitation that limits your choices.
Naturally, some foods have very strict requirements for agitation, heat transfer pattern, or both (e.g. a whipped cream needs tons of agitation, while a lasagna cannot have any), while others don't. If you know the requirements of a food, it doesn't matter what device you use to create them. If you don't know them, or know that the tolerable range is wide, best stick with the standard methods, they have become a standard for a reason.