The following is probably faulty. It uses a plugin and it may not actually emulate the functionality you seek...?
Reference: eLL
Code:

---

entity* ent0;
entity* ent1;
function getClosest(_what, &_centerPoint, _radius, &_point) {
	var dist0; dist0 = 99999;
	var dist1; dist1 = dist0;
	ent0 = NULL;
	eLLNextReset(_what);
	ent0 = eLLNext(_what);
	ent1 = NULL;
	while(ent0 != NULL) {
		if (vec_dist(_centerPoint, ent0.x) <= _radius) {
			dist1 = vec_dist(ent0.x, _point);
			if (dist1 < dist0) {
				dist0 = dist1;
				ent1 = ent0;
			}
		}
		ent0 = eLLNext(_what);
	}
	//ent1 should now be NULL or the closest thing of type _what to _point 
	/// also in sphere centered at _centerpoint with _radius?
}

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