The option of wearing a bra with a particular top and control it being visible when the top is full on is controlled by the parameters in the customlist of the particular top. So if you edit the list parameters for that top you can make it possible to wear a bra and see it all the time.
However there is a problem. In many cases the meshes of the tops and the meshes for the bras do not correctly align, specially around the breasts and nipples areas. So if you make the bra visible always you get a lot of clipping issues with bra parts sticking out of the top instead of being concealed. See the image below.
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Some tops go around this by displaying the bra only when the top is down in the half-dressed state, that way you don't get the clipping when the top is completely on. But I find that there are some tops which would look strange if you do this. For example in a strapless dress it doesn't make sense not to have the straps from the bra showing when the top is up and then having them suddenly appear when the top is down.
If you want to experiment let me know and I will tell you which parameters to change on the lists to get the effect.