If wmw provides more support from reputable dictionaries, then this might be worth considering. As it is, I would not support the suggestion.
With noun-verb pairs of this kind in English, it is common for a hyphen to be used in the nominal form (write-up), while the verb is open (write up). This is supported by the first two sources I checked, the AHD and Merriam-Webster online. http://www.merriam-webster.com/dictionary/wri...
write–up noun \ˈrīt-ˌəp\
Definition of WRITE-UP
1: a written account; especially : a flattering article
2: a deliberate increase in the book value of an asset (as to reflect the effect of inflation)http://www.merriam-webster.com/dictionary/wri...
write up transitive verb
Definition of WRITE UP
1: to make a write-up of
2: to report (a person) especially for some violation of law or rules
If the original poster has reputable sources that contradict established dictionaries and demonstrate that nominal "write up" is a valid, rather than a sloppy spelling, he could post links to them. Lacking that, I see little reason to put more time into this than he has.