There isn't any Transcend Armor option in current HARP rules. It's not hard to house-rule it, though. Or to find workarounds. For example:
In one of the old Harper's Bazaars (can't remember which one, but it's also in the HB Annual 2005), there is a Paladin profession which has its own spell list, and a talent that reduces armor PP penalties by 1 starting from 4th level.
This penalty reduction increases by 1 every 4th level after that (8th, 12th, 16th, etc). Applies to
ARMOR penalties only, and reduces the penalty to a minimum of 1.
(personally, i think it's a bit weak and would start it from 1st level. and allow it to reduce armor penalties to 0).
You could make a Talent that does something similar. Or give it as a freebie to Clerics who have Combat as one of their favored categories (the standard HARP rules classify them as "Paladins")
Another option is to have a skill called "Armored Casting" or similar that reduces the PP penalty. I've used a variant of this which reduces the PP penalty by 1 for every 7.5
ranks (so -1 PP at 8 ranks, -2 PP at 15). I've also used another variant which reduces the penalty by 1 for every 20 skill
bonus. e.g. a skill bonus of 40 would reduce the armor PP penalty by 2.
Either variation works OK without unbalancing things - it costs a
LOT of DP(*) , which could otherwise go into spells or other skills.
In both cases, they reduced the armor penalty ONLY (i.e. can not provide a bonus), to a minimum of zero.
(*) lots of ranks at 2 DP per rank. For the first variant there's a minimum of 8 ranks (16 DP, and minimum character Level 2) before the character sees any benefit at all. For the second variant, it's easy to get a +40 skill bonus at 1st or 2nd level or +60 by 2nd or 3rd level, but it's subject to seriously diminishing returns once you get 10 ranks in the skill due to the -25/5/2/1 skill rank progression.
And yet another option is to just have a house-rule that says the armor penalty increases PP cost but
doesn't affect casting rolls or increase the casting time - so, e.g., wearing Soft Leather makes each spell cost 2 PP more but doesn't impose a -10 penalty to casting rolls.
BTW, most caster classes have some variant of Mage Armor (Steel Skin, Tree Skin, etc). Battle Runes (from Martial Law) can be used to provide instant casting of unscaled Mage Armor, and either the Potion Mastery or Rune Mastery spells can be used to create scaled versions.
There are also magic items that provide a DB bonus without a casting penalty, and/or Daily use items that can cast Mage Armor.
The Cyradon book has an "Argentian Cuirass" steel breastplate as a special starting item option. Costs 25 DP, eliminates 3 PP of armor penalty.
Finally,
Armor by the Piece (AbtP) and high quality materials/craftsmanship allow a caster to get armor roughly equivalent to SL with no casting penalty, or RL with a penalty of 1 or 2 instead of 4. Exceptionally good materials can do even better than that.
e.g. here's what I call the "Mage Special":
Rigid Leather armor-by-the-piece Helm, Gorget, Gauntlets, Bracers, and Boots.
Do
NOT get Pauldrons, Cuirass, Greaves or Shirt unless you want to have a PP casting penalty on every spell (you don't).
This will give you from +15 to +23 DB (depending on whether you can spare ~15 gp of your starting gold for Improved and/or Superior Leather pieces). You'll probably need to spend some DP to buy 2 or 3 ranks of Armor Skill to bring the armor maneuver penalty down to the minimum (-3 for normal, 0 for Imp. SL).
If you're not buying Improved or Superior Leather armor, it's cheaper to buy a full suit of RL and leave the shirt, pauldrons and greaves at home (or sell them for whatever you can get).
For comparison, a full suit of Soft Leather is +20 DB and has a casting penalty of 2 PP. You
really don't want that.
If you want slightly better armor, add a Soft Leather shirt (adds another +8 DB for Normal, +13 for Imp. SL), and acquire a +1 PP adder to offset the 1 PP casting penalty.
AbtP combines well with an Argentian Cuirass