OSGi Tasklets contributed to DOTS are by default not visible to XPages. This is because the OSGi containers for Http and dots tasks are loading different set of plugins. You could contribute the plugin where your OSGi tasklet live to the Http OSGi, but it won't resolve because of missing dependencies. So unless we port the entire OSGit tasklet Container over to the Http task (which is technically possible but there are no plan at the moment to do that), you won't be able to run tasklets in XPages.
However, if all you need is trigger a long running task within XPages you can always use eclipse Jobs to do that. Of course, you won't get all the features provided by the Tasklet Container like scheduling, event triggering, built-in OSGi command to communicate with the framework, etc...
I would also add a work of caution not to overuse eclipse jobs within the Http task as each job runs in its own thread and you could potentially decrease performances.
I am also currently exploring different ways of letting XPages drive the tasklet container by using remote requests (see example in this release of OSGi Tasklet Service) but also other IPC means. I would love to hear some use cases on that in this forum.
-david