in the future state of things, anything is possible as long as someone makes it so. but in the current state of things, i think this is the least bad method to sort of make favorites searchable: https://forum.inaturalist.org/t/best-way-to-organize-a-species-wish-list/29570/8 (second half of the post).
in other words, the key is to get your favorites as a list of observation IDs. then from there, you can apply whatever additional filters you want on this set of observations:
https://www.inaturalist.org/observations?id=107928097,107935946,107091296,106643731,106601339,102863390,106160692,105828644,105830935,105303403,105021779,104836611,60049428,104431162,104439366,70238367,104144723,104150477,104034080,103982950,67191915,65980388,65741210,63688380,62667396,62546009,61914279,61290528,50155878,49313840,48424896,41089703,40123893,39549601,28485656,28485736,26333197,26050449,26050445,25718631,25697286,25441469,25438351,103090008,21439849,21413491,20976061,20968040,20976844,20289763,20219196,19526512,19199909,19191395,17890059,11205244,10123742,9525528,102959993,102807299,102751179,101000084,101912282,40809643,40575730,102650720,102283647,100337327,102154130,101689618,100822449,100796990,100729309,99962703,99851310,99684469,99442967,99397859,99315737,99278568,99070200,98946832,98929997,98863038,98847688,98258759,98183868,98078720,97996169,97488542,97411801,97123241,96660859,96542823,96460681,96058677,95939946,95934731,95744498,95594858,95520659,95240700,95018759,95005931,94305225,94144755,94060975,93687048,93547152,92665818,92655265,92307432,91918868,91603528,90956624,90358961,90257525,88799155,87855236,86972409,83881017,83752503,82796289,81986524,81846594,81669867,80815149,79897901,79897900,79347327,78151662,78016919,77699918,77600254,77483733,77483640,77420722,77193366,77091311,76913739,76858503,76532523,76497649,76354999,76086322,75735523,75405637,75270324,75153232,73487113,73382022,73256117,73158561,72980906,72867225,72756601,72668556,72746610,72070801,71864750,71864118,71780953,71783071,71782889,71783260,71747339,71687811,71606619,71349906,71288398,71199026,71111451,70990480,102404784,42059139,65984824,25503662,70549093,70600919,70363410,70300687,70174274,70176507,70012563,69806576,69853214,69613097,69549556,69293385,69301144,69171643,69074947,68957344,68854227,68856295,68846351,68754301,68595829,68592213,68538568,68478013,68029636,68041761,67923072,67566257,67549572,67496189,67454649,67389883,67220052,67232756,71355196,66806223,66692572,65798903,65434105,65294040,65179139,65144198,65142886,65005136,38054585,63462799,64655728,64336616,64140073,63930180,63864826,63796471,63529616,63387195,63305321,50271882,63307579,66084394,48418891,50540517,64844920,63225216,62983050,62544734,62465096,95804998,62171722,61745127,61294962,61289269,61278910,61203108,61078991,60480484,58817318,47884911,49423007,60393550,60304340,60291208,58576335,56307663,55131559,101869289,101687517,101090045,60393543,100584219,98364934,76081715,99053274,97510829,98181850,98085004,97123462,90361913,80576513,85077053,79103565,55899839,71643367
…
the more i think about it, the more i think the easiest implementation is going to be to add a &fave_by=[user_id] parameter to /v1/observations, which would then flow to the observations search (explore) and identify screens. then you could get rid of the separate fave screen (or just point links to a preset version of the explore screen that filters for &fave_by=[your id]). if implemented that way, then you could just filter for faves using whatever other parameters are available for the observation search.