If you have tasks that have no link to other project tasks, then it sounds like conflating both projects tasks with operation-type tasks, i.e., your team is matrixed to both the project as well as daily operations.
If I interpreted this correctly, then I would argue that you would not use a scheduling tool like MSProject to capture operation tasks. If there is an expectation that your team will be bouncing back and forth from the project to operations, then you would load them less than 100% in the schedule and then direct and schedule them to perform operational tasks separately from the project schedule, i.e., calendar with stated duty times.
If you must show those operational tasks in the project schedule, then I would load them as LOE tasks--which they are--across time and load each team member to those tasks and assign some low level of utilization such as 10% and distribute those hours equally across time. This way, you will capture costs, both planned and actual, within the same tool. However, this will require manual assignment and I think this SHOULD require manual assignment as delegating this to a tool would not be advisable. There are considerations that you, as the manager, must make that no tool can analyze.