With the sale of Adam King to Swansea and deactivation of goalkeeping coach Alan Combe's player registration, Locke was hoping to bring in two players under the "one-out, one-in" sanctions stipulated in their transfer embargo.
However, any transfer coming in would still have to be agreed upon by an SPFL committee, and the panel has decided that Skacel cannot rejoin Hearts for a third time.
"I'm absolutely gutted about that," Locke said, "though to be honest I'm more gutted for Rudi.
"All he wants to do is play football and do that for Heart of Midlothian. He's been prepared to come here and play for free. It was nothing to do with finances. Now he's been denied that opportunity.
"I don't know who the board is. They've made a decision and I just have to take it on the chin.
"I'm not happy about it, but I don't run the game in this country. It doesn't matter what I say or I think. All I can say is that I'm disappointed."
Happier news for the Hearts boss was the go-ahead for McCallum's short-term loan from West Ham. The 20-year-old striker will go straight into the squad for Sunday's semi-final with Inverness.
"He's a big lad, exactly what we're needing," said Locke. "Obviously we started the season with two young strikers and not really any recognised first-team strikers. He'll give us another option up there.
"He's come with a high recommendation. I've spoken to many people I know in the game, people I trust, and they could not speak highly enough of him. I'm delighted to get him."
The manager remained tight-lipped on whether the recent acquisition would start the match, but admitted the signing, along with the return of Ryan Stevenson, would give him a selection headache, particularly as Hearts enter the game off the back of successive league victories for the first time this season.
"It's a great headache to have," said Locke. "I've been delighted with the last month. The players have got what their performances deserved.
"We're not getting carried away, it's two wins, but I think there's been a big difference in confidence.
"Guys are wanting the ball now, whereas before there was a nervousness in our passing. It's great going into such a big game.
"It's going to be difficult. Inverness have beaten us twice this season and they're a good team. If we play like we have in the last month then we'll give ourselves a chance."