I think the wording is getting in the way. What you want is a power supply that is voltage regulated, current limited.
A single LM317 can do this with a transistor for feedback to limit the charging current. Try this one, with a 5.6 ohm resistor in place of R1:
http://www.sentex.ca/~mec1995/circ/labc3.html
Keep in mind that that website uses 2.45V per cell. That is only good for quick charging, short term connection. For long term slow charging such as what you are doing, use 2.35V per cell.
So without a load, adjust for 6x2.35 = 14.1V as you have said. When the voltage drop across R1 reaches about 600mV, the transistor begins turning on, reducing the voltage, which decreases the charge current. That will be slightly greater than 100mA.
The charge curve: Initially, with current limited to 100mA, voltage across the battery will be less than 14.1V. As it charges, eventually voltage rises to 14.1V and charge current begins dropping.
Keep in mind that 100mA is not a float charge. If you keep it connected for long periods, it will damage a lead acid battery. Float charge is done at a constant voltage lower than the charge voltage, current should be on the order of 1/100th the Ah (amp hour) rating or less.