I almost thought this would be the reason. Yes, Offlinesurvey.com needs to run as all in one, which is of course not ideal. I don't think my solution will work in an all-in-one setting.
Here in South Africa, we have load-shedding (stages during the day when there is no power) and it also affects internet connections. We have to run the survey on an offline app during these stages and sync it when the networks are available again.
Luckily this becomes less and less relevant for us here in Brazil, at least in the more urban areas. Power outages can happen, but then it is usually due to a storm or very specific occurrences and then usually research needs to stop anyway.
Now in more rural areas this is still relevant and then offline apps are needed.
A couple of years ago I ran a webserver on my smartphone for test purposes and it actually worked quite well. Of course, nothing near as comfortable than an app, but as it was a full PHP/MYSQL stack it could run Limesurvey just like a normal web application. Of course, the effort to install and maintain LS as a separate installation on each device is less than ideal.
Maybe check with the developers of Offlinesurveys.com (there is a specific subforum for that here) if they have a solution for what you are trying to do. But I think in your case, there are workarounds that have worked over centuries when questionnaires where still paper and pencil and I think for your specific case, the "birthday" solution is not worse than the the random number and is a proven method.
Of course, this might not be what the client has asked for, but it works.
