There is a attachment you you can by for the tach cable at the distributer but haven't seen what you are askin
You don't have to take apart the dash to sevice the speedo cable, just a little bloody
If you have air conditioning you have to remove the lower duct (PITA)that passes over the steering colum. All you have to do is get you self on your back with your head under the dash and fanagle you hand up directly above the steering colum and depress the spring clip aganst the back of the speedo (towards the speedo) and work the cable off of the speedo with your other fingers (this is where the scraping thing happens). when cable is disconnected simply pull out the inner cable (with a pair needle nose pliers) clean,lube and re-install (you don't have to depress the spring clip when re-installing just make sure to spin the inner cable until it seats itself back into the transmission gear before hooking back to speedo)
Check with your local garage. The shop I used to work at had a small device that you screwed the transmission side of the speedo cable into, it was air pressurized and hooked to a drill, you spun the drill and the air pumped grease into the cable.
If you use a tool that lets you pump grease from the bottom, 2 things. 1, it's easy to put too much grease in the cable and it gets into the speedo head, then you get to have it rebuilt. 2, make sure you use the correct grease. Grease that's too heavy is as bad as, if not worse than none.