I know there are a lot of reasons (and probably a bunch more I don't know) why this is totally impractical but it would be kinda funny if at some point in the future humanity wants to get a robot presence in some other star system ASAP and is like "oh, speeding up an autonomous craft to near-light-speed is too hard, let's instead find some kinda planet or asteroid or so near our target that has the right kinda materials on it and blast it with enough light ('light' in the sense of something on the EM spectrum) in the right places to create the thing we need"
conversely it would also kinda suck if earth got wiped out because some aliens wanted to have a computer around this area and their remote litography tools were a bit too coarse to be survivable
The diffraction limit might imply that your sending antenna has to be impractically large (in diameter). Some Greg Egan's stories include a variation of that, where the thing that's accelerated is not a craft of any sort, but something that will in a few milliseconds from being launched (onboard time) start constructing the desired craft out of the material of the moon/star it has just impacted into. One example of a story of that shape: https://www.gregegan.net/INCANDESCENCE/00/Crocodile.html (I can't find the story that I remember having a closer approximation to what you described.)
