You can cross reference just the individual member - depending on how you get to the cross reference window, you may have to manually add it to the end of the cross reference "search" field.
However, this will not only return an explicit reference to just that element - it will also return any logic that may act on that element in an indirect manner.
To take your example of cross referencing a timer done bit. If you do this, you will get a list of everywhere the Timer.DN bit is used on an XIC, XIO, or whatever other creative instruction you have addressed using the Timer.Dn bit. But, you will also get the rung that has the TON or RTO instruction on it as well. Why? Because even though your Timer.DN bit is not explicitly addressed on this rung, the TON instruction uses the .DN bit internally, and has the power to modify it.
You will see the same concept if you cross reference a member of an AOI - you'll always come up with the rung containing the actual AOI itself as well, for the same reason.
The same holds true for a UDT - if there is any operation that acts on your UDT as a whole, the element you're cross referencing is relevant to that logic. It makes sense when you think about it - if I want to see where this data comes from, I need the cross reference to find a COP instruction acting on the whole UDT, not just a MOV instruction moving one specific value into that particular element. Likewise if I'm trying to see all the places the data goes, if there's a blanket COP instruction copying my whole UDT to somewhere else, I want to find that too.