Yes, you can thread without a threading dial or a reversing switch. It is called the "sharpie method". You will need two registration marks. One is not sufficient. One mark is on the chuck backplate, registering with the headstock. The second is on the carriage, registering with the ways. This seems too good to be true. How can just two sharpie marks keep the engagement of the half nuts in time with the chuck. No markings are necessary on the gears. In fact, it is best to keep the gear cover shut while threading, for safety reasons.
Why does this work? Well, lets do a little thought experiment. Suppose the half nuts are engaged right as the tool touches the workpiece. This won't happen, of course, since you need a little runway for comfort, but let's consider this situation anyway for simplicity. If you disengage the half nuts and roll the carriage back to the starting position, then start the lathe and engage exactly when the chuck sharpie marks line up, the workpiece will be at the correct place for the thread to start, and so will the tool, because it will have been rolled back to exactly that position. The threading tool must enter in the correct place. There is no other choice. If the lead screw is at the wrong "time" when the chuck marks are aligned, the half nuts will refuse to close. Do not force them! Just turn one more rotation, and see if they will close. They will eventually close. At that point, the tool will start cutting precisely at the entry point of the last cut. If you only have one set of marks, this won't be the case. You can get a situation in which the half nuts engage when the tool is at the wrong angular location with respect to the workpiece, and it will dig a new (unwanted) thread.
Maybe this hand-waving argument did not convince you. In that case, you can try it a few times, and that will convince you. If you are particularly skeptical (and I don't really like very skeptical people), you can try the following confidence builder. You will need two sharpies. Yes, I know, sharpies aren't cheap. But, only one of them has to be sharp and new. The second can be blunt and barely functional. If you are having difficulty obtaining two sharpies (I've been there, so don't stress), wait until you see them at a garage sale. A great opportunity is a teacher liquidating sharpies from the classroom. You can get a whole box for a few bucks. Most will be bad, but you only need one sharp nice one. Note that they don't have to be name brand. Bic brand will also work. Fasten the good one into the tool holder in place of a tool. Run through the process. In this case, it is safe to start right at the workpiece, since sharpie tips do not fracture if you accidentally ram them into the part like carbide inserts. Check for double tracking on repeated re-engagements. By carefully watching the carriage and the chuck positions, you will quickly become convinced of the soundness of this method. A good workpiece is a piece of white PVC pipe. You can usually obtain one of these for nearly free as an offcut, and visibility of sharpie marks of nearly any color is excellent. A black sharpie is not necessary to do this check. Also, one problem is that the mark on the ways may be difficult to place or even see if the ways are very oily. In that case, scrub the spot vigorously with a wadded up piece of paper. Cheap paper contains a lot of clay and silica and is slightly abrasive. Don't worry about ruining the accuracy of your lathe, because the place you will shine up is not a bearing surface. The blemish will be cosmetic only, as will the sharpie marks from your threading jobs. If too many sharpie marks are present due to a lot of threading, they are easy cleaned with solvent, or maybe it is time to get that reversing switch or threading dial.
