More specific detail would be helpful.
When you say 'mid-line', this can be in 2 different configurations as follows:
1. as a single rope segment; and
2. as a doubled rope segment.
Which one are you thinking of?
In terms of the doubled rope segment, an example is where a lead climber ties into both ends of the rope to lead a short pitch (double rope trad climbing). The 'second' climber would tie-in using the 'bight' that is actually the mid point of the rope.
I recommend 'Scotts locked Bowline' (tied doubled).
In the case of a single rope segment, this would be a third climber tying-in to the mid point of the rope (to make a party of 3).
In this example I recommend #1074 Bowline with a bight. You can even tie 2 'Bowlines with a bight' if force can come bi-directionally.
Note that in both cases, no carabiners are required...