I want to calculate the implied repo to a delivery date for a series of bonds that have not been issued yet. Do I make assumptions about the bond yields on some trade settlement date and the futures invoice price?
There are two assumptions involved – the coupon rate and the yield. Everything else – maturity date and issue date – is known. Once these two assumptions are set, calculating implied repo rates is trivial.
Setting these two assumptions is of course an art. Overall, it's not all that different from analyzing any bond auction – you calculate a yield spread to the current on-the-runs, accounting for curve, liquidity premium, carry, and bad days. Most strats prefer doing this as of the delivery date; i.e., we'd "guess" a forward yield spread relative to the on-the-run issue.
It can be simpler than that. Just create custom hypothetical bonds on Bloomberg and then load up a futures contract. For example, TUZ8 is the December two year futures contract. Once you bring that up, add the custom bonds and then just play around with the yields shifting the spreads between the current CTD and the hypothetical bonds. They show the implied repos too on the screen.