I have always run model t ign total loss from battery.I have always been curious if a modern coil could be used with the powers connected to battery power, the negatives to their respective timer contact and the spark output the the correct spark plug. I would think that a condensor would need to be wired into the negative side of each coil and also that the best timer for this would be the flapper type. This is basically what triumph motorcycles had for ign and they rev to over 5 thousand rpm. I realize this may send sparks out around this forum but have fun with it!
It would be possible to make it run that way, but it would run poorly on battery and not at all on MAG.
Problem is the modern coils need a break in the primary to fire and the Model T connect to fire.
The problem with the ignition configuration you describe is that the stock timer establishes and maintains ground contact for approximately 90 crank shaft degrees of rotation which is way too long to serve as the mechanism of charging and discharging the coil, especially at low RPM. The coil will saturate and draw excessive current (>15A) from the battery and risk overheating or damaging the coil. Spark will occur when the timer moves off contact, but will not occur at the proper time unless special means are used to adjust the timer rotation and it will produce a heck of an arc at the timer contacts. A condenser connected between timer contact to ground may help but defiantly not a good method of ignition.
Operation on magneto may actually be more workable since the magneto output varies with time. The coil will charge and discharge 4 times during timer contact to ground but would only produce spark if the timer moves off contact at the proper time. The timer would also have to be positioned such that it moves off contact during one of the 4 magneto cycles when the firing cylinder is in the proper position. Coil saturation/overheating/damage is still an issue because the magneto output varies with engine RPM.
The original Model T ignition system is a simple and elegant arrangement. The coil points function to self-regulate the coil dwell time and produce spark when they are properly adjusted irrespective of how long the timer remains in contact with ground. The timer functions as intended; to control ignition timing.
Mike K. should know for sure but I would guess that because the points are bypassed and not needed on an E-Timer, then modern coils should work when using an E-timer. I don't know if the E-Timer relies on the condenser in the original coil to reduce the load on the E-Timer.
The TruFire ignition uses modern coils with the original timer and some electronics, I would guess that with the proper condenser/electronics that a person could get stand along modern coils to work.
Bottom line, if it was really simple to do, then it would have been done in the past. The original system is quite simple and reliable.
I've been making electronic T coils for a while now. Here's a previous thread about them: