I have a 300mm seatpost which drops all the way into the frame. If I buy a 380mm reverb will it be ok?

Yes.

As above-Yes
Mind if you has bottle bosses fitted on the seat tube with the bolts in them,being long they’d impead the post if it went down that far.

A 380 post will reduce to 280 or 255 depending on travel when dropped so will be fine, however If you’re running a 300mm post, I’d be more worried about how much seatpost you have out of the frame when your saddle is at pedalling height. A 125mm drop reverb measures 190mm from the bottom of the collar to the centre of the saddle rails. If you’ve got less than this then you need the 100mm drop. If you’ve got less than 165mm then a reverb will always return to a position too high for pedalling.