It is probably not worth it in most cases but this is doable.
I managed to get an app running after quite a bit of work - the javascript support is really not great in OS 5.0 and debugging is very very slow as suggested in bfcapell's answer.
To get backbone to work you need to comment out the code that uses the hashchange event to handle url changes (this is assuming that the router is being used). THere is a fallback in backbone which uses setinterval to poll for changes.
// Depending on whether we're using pushState or hashes, and whether
// 'onhashchange' is supported, determine how we check the URL state.
/*if (this._hasPushState)
{
alert('pushstate');
$(window).bind('popstate', this.checkUrl);
} else if (this._wantsHashChange && ('onhashchange' in window) && !oldIE)
{
alert('hashchange');
$(window).bind('hashchange', this.checkUrl);
} else if (this._wantsHashChange)
{*/
this._checkUrlInterval = setInterval(this.checkUrl, this.interval);
//}
The foreach method in underscore also needs to be modified to not use the native foreach method. This is needed for collections to be rendered correctly.
var each = _.each = _.forEach = function (obj, iterator, context)
{
if (obj == null) return;
/*if (nativeForEach && obj.forEach === nativeForEach)
{
obj.forEach(iterator, context);
}
else*/
if (obj.length === +obj.length)
The above should get at least backbone mostly working. (I say mostly because I have a completely working app but I suspect to find a couple more OS5 specific issues in time).