i've never really read the extended lore, so i dunno
I always assumed halo's idea of slipspace, ships have to choose to travel through and can decide to stay inside of it as long as they want. so you could sending something into slipspace and just.. not have it move.
edit: my joke post seems to not hold
http://halo.wikia.com/wiki/Slipstream_Space
"Slipstream space is a specific set of seven dimensions existing in a very small bundle. While these dimensions are present in normal space, they do not have an effect on the physics of normal space."
basically space magic and i'm too lazy to link to the 'correct' wiki.
"The Forerunners had a far greater understanding of Slipstream space, with the abilities to travel nearly instantaneously over galactic distances, to disrupt slipstream travel from normal space, to create bubbles of Slipstream space, accelerating time, slowing time considerably inside (or even a complete time lock) while keeping the contents of the bubble visible in normal space, to store considerable mass and volume in Slipstream space stably for thousands of years and potentially for all of time, and to transition matter from normal space to the inside of a construct in Slipstream space without requiring the construct to transition back to normal-space."
so basically the Forerunners could make slipspace do whatever the hell they wanted it to.