Getting a train seat reservation without a ticket

Yes you can.

You can also buy your tickets any time you want as well, even before you have the Half Fare Card.

Note that the Golden Pass Express does no require reservations, but in the summer they are a good idea anyway. Note also that if you have any other travel before or after the GPX you can get a through ticket. So eg if you were planning to travel from Lauterbrunen to Vevey via the GPX you would get:
- A ticket Lauterbrunnen - Vevey via Zweisimmen
- A reservation Interlaken Ost - Montreux on the GPX

And with the HFC you do get a reduction on the ticket

You don't have to buy the ticket now, so you have some time to consider what pass you may end up buying,