It is possible to ease the length back to where it should be.
Take your length of tape & mark the intended length on it with pins, also put a pin in the 1/2 way mark, 1/4 & 1/8. On the bias piece pin mark the 1/2, 1/4, 1/8. Match up the pins starting with the 2 ends then the centre. Once you have got the 1/8 sections pinned, continue finding the centre between 2 pins of both layers and pin them together. You will need to do this until your pins are about 1/8" apart (maybe 1/16" apart depending on the amount to be eased). At this point you could lay it on a flat surface, dampen it (with spray starch) and pat it back to flat (or iron it) or you could stitch on the tape, & remove the pins before you dampen & flatten it.
The technique (minus the starch) is used in couture dressmaking to tighten up necklines & armholes, & I have used it to great effect in this way, But there is no reason that it won't work for you - you just need 'millions' of pins and a bit of time & concentration.