One small issue with this request is to decide the behaviour of when the default tags get sent. The original request suggests that the default tags would only be used for reports that are sent automatically, so that they can be distinguished from reports that are sent manually. But you've pointed out another good but different use-case where you'd want default "Android" or "iOS" tags to be included with both automatic and manual reports.
One way for this to work both ways is for the basic Send method to report any specified default tags, and then the Send overload that takes tags could use the specified tags parameter and ignore the default tags. But then some people may want default tags to be included with manually sent tags.
We'll have a think about this, and any further feedback would be great.
Another solution that is available right now is to attach an event handler to the RaygunClient.Current.SendingMessage event. In this event handler, you can modify the e.Message.Details.Tags value. This will let you set "Android" or "iOS" tags for any message being sent to Raygun - both automatic and manual. This also give you control of whether you want to append the tag(s) to the list, or override any manually specified tags.