i don't pitch modified but i have been warming up with a windmill motion ever since i was 20 and i got tendonitis in my elbow... i always start out throwing about 15-20 fast pitches because it loosens my shoulder and elbow up slower... so i know for a fact this slider works with that delivery... for anyone who hasn't seen my thread on the slow pitch slider yet...
for the modified pitchers, the same physics that i use to get my slider in slow pitch can be used here as well... in fact, reading some descriptions of the modified curves used in this thread i find most like the 2 seam fastball grip which means you have the smooth spot in between your thumb and index finger... in other words, you're already using this and don't know it... if you think it's curving because you impart a lot of spin then i ask... with this grip, you only have 2 seams involved as the ball rotates... shouldn't you get more curve with 4 seams rotating through the air? that's what all the spin is for... so the seams create the magnus effect... if you throw this same pitch so that 4 seams are involved... do you get more curve? or no curve? i predict no curve because the smooth spot won't be oriented correctly that way...
try making a bullseye ball and see if it's true... if so... then you can refine the pitch as well as change the amount of curve you get by merely rotating the ball a little in your hand... also... keep in mind... with slow pitch the smooth spot that's used to get the slide is between your thumb and index finger... with fast pitch or modified the smooth spot you will be using will be on the reverse side where your pinky is... so when you grip the bullseye ball, the offset bullseye should be the one you can see...
if you're looking to develop a curve [slider] then follow the instructions in the slow pitch thread keeping in mind you are putting the opposite spin on it from slow pitch... also remember... the amount of spin is less important than the orientation of the smooth spot... this should also make it easier to throw acurately... and lastly... put the front bullseye on the opposite side...
also... with this technique... in theory, you should be able to get movement in any direction if you orient the smooth spot and the seam correctly... just something else to ponder...
here's the link to the slow pitch slider thread... let me know if i can help since, even though i don't pitch modified, i do have experience thowing this pitch...
http://forums.softballfans.com/threads/pitchers-slow-pitch-slider.1447114/