yes, limited space on EX2300 and EX3400 is a known problem. There have been some threads in this forum, but I'm only able to dig up one about EX3400 right now (the situation for EX2300 is similar AFAIK).
Juniper has documented the issues in KB31198 and offers a guide for "troubleshooting". This also mentions the unlink argument, so I think Nexon is wrong here that it's not supported on EX.
For our three EX3400 in VC, we have a USB stick in the master device and download the update to there. Using that space extension, yesterday's update to the latest 18.1R3-S7 has worked without problems (with said unlink flag). In case that doesn't help, you can try deleting old packages that would be overwritten by the update anyhow:
Just FYI for all, the situation surrounding both Upgrades and Snapshots, etc. are well-known internally with in Juniper TAC and Engineering, and work is on-going to get all of this resolved in an upcoming SW release.
At this time vechicle (SW release number) nor timeframe is known, but this is a high priority situation that is being work on.