Cool idea, but that would require a lot of money and is not technologically feasible currently, given that synthesized DNA can only be synthesized up to a couple thousand base pairs as limited by the current technology. Additionally, getting such a short DNA molecule to self replicate may prove difficult.

Another idea is to do polymerase chain reaction (PCR) using isotope labeled dNTPs (the nucleotides A, T, G, and C). Do one round of PCR, save that product, do a second round, save that product, etc until you have enough rounds of PCR (analogous to generations of E. coli) to do your very own Meselson-Stahl inspired experiment!