Portaloos have existed for a long time, we can use that data.
Unless too many people start using it, there won't be any difference from a normal portaloo.
Under International Building Code in the US (it's pretty similar to the one in UK too), single-occupancy and accessible toilets are already regulated for safeguarding, like doors to be fitted with emergency release mechanisms (like coin-turn locks) that allow external access by staff or emergency services or that doors must open outwards.
Based on CPTED, we don't even have to worry about loitering zones, and the access control is binary.
There isn't really data about portaloos, but we can look into the broader umbrella term for them, (which is what I'm generally advocating for, I just listed the cheapest version of them)
Washington, D.C. which passed a municipal regulation requiring all single-occupancy public restrooms to be gender-neutral in 2006.
ohr.dc.gov/page/safe-bathrooms-dc?hl=en-GB
The actual danger in the city was not the single-occupancy rooms themselves, but the lack of safe access to them.
And Seattle, which did the same in 2015.
content://com.google.android.apps.docs.fetcher.FileProvider/ida10GNBdtvZyNiVPlV6ONPgm0N8eC3txpy70d9BlTI%3D?_display_name=2015%20SOCR%20AR%20single%20final.pdf&mime_type=application%2Fpdf&_size=12803910
There was no recorded spike in violence and local advocacy groups reported a positive effect on safety.
There were 73 reports, but these were of things like businesses failing to update their signs.
www.parliament.nsw.gov.au/lcdocs/inquiries/3071/Report%20No.%204%20-%20Portfolio%20Committee%20No.%208%20-%20Customer%20Service%20-%20Public%20toilets.pdf?hl=en-GB
Says single-use cubicles that open directly onto public space are the preferred design over multi-stall gendered blocks
www.cambridge.org/core/journals/behavioural-public-policy/article/genderneutral-bathroom-a-new-frame-and-some-nudges/C6CDCA42BAEBCE684B243EB9773A771C?hl=en-GB
The shift toward single-stall bathrooms has been formally proposed as a method to reduce violence in public restrooms and combat bullying in schools.
content://com.google.android.apps.docs.fetcher.FileProvider/93wG5sM8Fv9WItOVQSyzsGPkSWtsBneRFuU43SzTKLQ%3D?_display_name=APSC%20262%20-%2002%20Washrooms%20-%20AL%20KM%20ST_0.pdf&mime_type=application%2Fpdf&_size=696859
This goes into vandalism repair costs in single-stall units, which should offer a good idea regarding portaloos too. (UBC APSC 262)